/* ============================================
   ML Guide — Premium Design System v2
   ============================================ */

/* ---- Design Tokens ---- */
:root {
  /* Primary indigo-violet system */
  --primary:        #6366f1;
  --primary-dark:   #4f46e5;
  --primary-light:  #818cf8;
  --primary-glow:   rgba(99, 102, 241, 0.25);

  /* Secondary */
  --secondary:      #8b5cf6;
  --secondary-dark: #7c3aed;
  --secondary-light:#a78bfa;

  /* Accents */
  --accent:         #06b6d4;
  --warm:           #f97316;

  /* Category palette */
  --cat-data:       #06b6d4;   --cat-data-bg:   rgba(6,182,212,0.12);
  --cat-prep:       #10b981;   --cat-prep-bg:   rgba(16,185,129,0.12);
  --cat-eda:        #f59e0b;   --cat-eda-bg:    rgba(245,158,11,0.12);
  --cat-ml:         #6366f1;   --cat-ml-bg:     rgba(99,102,241,0.12);
  --cat-dl:         #8b5cf6;   --cat-dl-bg:     rgba(139,92,246,0.12);
  --cat-opt:        #f43f5e;   --cat-opt-bg:    rgba(244,63,94,0.12);
  --cat-mlops:      #14b8a6;   --cat-mlops-bg:  rgba(20,184,166,0.12);

  /* Difficulty */
  --diff-beginner:     #22c55e;
  --diff-intermediate: #f59e0b;
  --diff-advanced:     #f43f5e;

  /* Surfaces (light mode) */
  --bg:        #f4f4f5;
  --surface-1: #ffffff;
  --surface-2: #fafafa;
  --surface-3: #f0f0f2;
  --surface-4: #e4e4e7;

  /* Text */
  --text-1: #09090b;
  --text-2: #3f3f46;
  --text-3: #71717a;
  --text-4: #a1a1aa;

  /* Borders */
  --border:       #e4e4e7;
  --border-strong:#d4d4d8;

  /* Gradients */
  --gradient-primary: linear-gradient(135deg, #6366f1 0%, #8b5cf6 100%);
  --gradient-hero:    linear-gradient(135deg, #0d0d1a 0%, #1a0d30 50%, #0d1a30 100%);
  --gradient-glow:    radial-gradient(ellipse at 50% 0%, rgba(99,102,241,0.18) 0%, transparent 65%);

  /* Glass */
  --glass-bg:     rgba(255,255,255,0.72);
  --glass-border: rgba(255,255,255,0.55);

  /* Code */
  --code-bg:   #09090b;
  --code-text: #e4e4e7;

  /* Typography */
  --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
  --font-mono: 'Fira Code', 'Monaco', monospace;

  /* Spacing scale */
  --sp-1: 0.25rem;  --sp-2: 0.5rem;  --sp-3: 0.75rem;
  --sp-4: 1rem;     --sp-5: 1.25rem; --sp-6: 1.5rem;
  --sp-8: 2rem;     --sp-10: 2.5rem; --sp-12: 3rem;
  --sp-16: 4rem;    --sp-20: 5rem;

  /* Layout */
  --sidebar-w:   272px;
  --header-h:    64px;
  --content-max: 860px;

  /* Radius */
  --r-sm:   6px;
  --r:      10px;
  --r-lg:   16px;
  --r-xl:   24px;
  --r-full: 9999px;

  /* Shadows */
  --sh-sm: 0 1px 3px rgba(0,0,0,0.07);
  --sh:    0 2px 10px rgba(0,0,0,0.09);
  --sh-md: 0 4px 20px rgba(0,0,0,0.11);
  --sh-lg: 0 8px 36px rgba(0,0,0,0.13);
  --sh-xl: 0 20px 60px rgba(0,0,0,0.16);
  --sh-glow: 0 0 36px rgba(99,102,241,0.22);

  /* Transitions */
  --t-fast:   150ms cubic-bezier(0.4,0,0.2,1);
  --t-smooth: 280ms cubic-bezier(0.4,0,0.2,1);
  --t-bounce: 420ms cubic-bezier(0.34,1.56,0.64,1);
}

/* ---- Dark Mode ---- */
[data-theme="dark"] {
  --bg:        #09090b;
  --surface-1: #18181b;
  --surface-2: #1c1c1f;
  --surface-3: #27272a;
  --surface-4: #3f3f46;

  --text-1: #fafafa;
  --text-2: #e4e4e7;
  --text-3: #a1a1aa;
  --text-4: #71717a;

  --border:        #27272a;
  --border-strong: #3f3f46;

  --glass-bg:     rgba(24,24,27,0.82);
  --glass-border: rgba(63,63,70,0.55);

  --sh-glow: 0 0 48px rgba(99,102,241,0.35);
}

/* ============================================
   Reset & Base
   ============================================ */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

html { font-size: 16px; scroll-behavior: smooth; -webkit-font-smoothing: antialiased; }

body {
  font-family: var(--font-sans);
  font-size: 1rem;
  line-height: 1.65;
  color: var(--text-1);
  background: var(--bg);
  min-height: 100vh;
}

a { color: inherit; text-decoration: none; }
img { display: block; max-width: 100%; }
button { font-family: var(--font-sans); cursor: pointer; }

/* ============================================
   Reading Progress Bar
   ============================================ */
.reading-progress {
  position: fixed;
  top: var(--header-h);
  left: 0;
  width: 0%;
  height: 3px;
  background: var(--gradient-primary);
  z-index: 200;
  transition: width 0.1s linear;
  border-radius: 0 var(--r-full) var(--r-full) 0;
}

/* ============================================
   Header
   ============================================ */
.header {
  position: fixed;
  top: 0; left: 0; right: 0;
  height: var(--header-h);
  background: var(--glass-bg);
  border-bottom: 1px solid var(--border);
  backdrop-filter: blur(16px) saturate(180%);
  -webkit-backdrop-filter: blur(16px) saturate(180%);
  z-index: 150;
  transition: box-shadow var(--t-smooth);
}
.header.scrolled { box-shadow: var(--sh-md); }

.header-content {
  max-width: 1500px;
  margin: 0 auto;
  padding: 0 var(--sp-6);
  height: 100%;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

/* Logo */
.logo {
  display: flex;
  align-items: center;
  gap: var(--sp-2);
  font-weight: 700;
  font-size: 1.15rem;
  color: var(--text-1);
  transition: opacity var(--t-fast);
}
.logo:hover { opacity: 0.85; }
.logo-icon {
  width: 32px; height: 32px;
  background: var(--gradient-primary);
  border-radius: var(--r);
  display: flex; align-items: center; justify-content: center;
  flex-shrink: 0;
}
.logo-icon svg { width: 18px; height: 18px; color: #fff; }
.logo-text {
  background: var(--gradient-primary);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

/* Header Actions */
.header-actions { display: flex; align-items: center; gap: var(--sp-3); }

.search-form { display: flex; align-items: center; }
.search-input {
  width: 230px;
  padding: var(--sp-2) var(--sp-4);
  border: 1px solid var(--border);
  border-radius: var(--r-full) 0 0 var(--r-full);
  font-size: 0.875rem;
  outline: none;
  background: var(--surface-1);
  color: var(--text-1);
  transition: border-color var(--t-fast), box-shadow var(--t-fast);
}
.search-input:focus {
  border-color: var(--primary);
  box-shadow: 0 0 0 3px var(--primary-glow);
}
.search-input::placeholder { color: var(--text-4); }
.search-btn {
  padding: var(--sp-2) var(--sp-3);
  background: var(--gradient-primary);
  color: #fff;
  border: none;
  border-radius: 0 var(--r-full) var(--r-full) 0;
  display: flex; align-items: center; justify-content: center;
  transition: opacity var(--t-fast);
}
.search-btn:hover { opacity: 0.88; }
.search-btn svg { width: 16px; height: 16px; }

.icon-btn {
  width: 38px; height: 38px;
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r);
  display: flex; align-items: center; justify-content: center;
  color: var(--text-3);
  transition: all var(--t-fast);
}
.icon-btn:hover { color: var(--primary); border-color: var(--primary); background: var(--primary-glow); }
.icon-btn svg { width: 18px; height: 18px; }

/* Mobile search icon (shown on small screens) */
.search-icon-btn { display: none; }

/* ============================================
   Layout
   ============================================ */
.container {
  display: flex;
  min-height: 100vh;
  padding-top: var(--header-h);
}

/* ============================================
   Sidebar Navigation
   ============================================ */
.sidebar {
  position: fixed;
  left: 0;
  top: var(--header-h);
  width: var(--sidebar-w);
  height: calc(100vh - var(--header-h));
  background: var(--surface-1);
  border-right: 1px solid var(--border);
  overflow-y: auto;
  overflow-x: hidden;
  z-index: 90;
  scrollbar-width: thin;
  scrollbar-color: var(--border) transparent;
}
.sidebar::-webkit-scrollbar { width: 4px; }
.sidebar::-webkit-scrollbar-track { background: transparent; }
.sidebar::-webkit-scrollbar-thumb { background: var(--border); border-radius: var(--r-full); }

.nav-toggle {
  display: none;
  position: fixed;
  top: calc(var(--header-h) + var(--sp-3));
  left: var(--sp-4);
  z-index: 95;
  background: var(--gradient-primary);
  color: #fff;
  border: none;
  padding: var(--sp-2) var(--sp-3);
  border-radius: var(--r);
  box-shadow: var(--sh-md);
}
.nav-toggle svg { width: 20px; height: 20px; }

.nav { padding: var(--sp-4); }
.nav-home-link {
  display: flex;
  align-items: center;
  gap: var(--sp-3);
  padding: var(--sp-2) var(--sp-3);
  border-radius: var(--r);
  color: var(--text-3);
  font-weight: 500;
  font-size: 0.9rem;
  transition: all var(--t-fast);
  margin-bottom: var(--sp-2);
}
.nav-home-link:hover { background: var(--surface-3); color: var(--text-1); }
.nav-home-link.active { background: var(--primary-glow); color: var(--primary); }
.nav-home-link svg { width: 18px; height: 18px; flex-shrink: 0; }

.nav-divider {
  height: 1px;
  background: var(--border);
  margin: var(--sp-2) 0 var(--sp-3);
}

.nav-section { margin-bottom: var(--sp-1); }

.nav-category-btn {
  display: flex;
  align-items: center;
  gap: var(--sp-3);
  width: 100%;
  padding: var(--sp-2) var(--sp-3);
  border-radius: var(--r);
  color: var(--text-2);
  font-weight: 600;
  font-size: 0.85rem;
  background: none;
  border: none;
  text-align: left;
  transition: all var(--t-fast);
  position: relative;
}
.nav-category-btn:hover { background: var(--surface-3); color: var(--text-1); }
.nav-category-btn.active { color: var(--primary); background: var(--primary-glow); }

.nav-cat-icon {
  width: 28px; height: 28px;
  border-radius: var(--r-sm);
  display: flex; align-items: center; justify-content: center;
  flex-shrink: 0;
  font-size: 0; /* hide text fallback */
}
.nav-cat-icon svg { width: 15px; height: 15px; color: #fff; }

.nav-cat-label { flex: 1; }

.nav-count {
  font-size: 0.7rem;
  font-weight: 600;
  padding: 1px 6px;
  border-radius: var(--r-full);
  background: var(--surface-3);
  color: var(--text-4);
  margin-right: var(--sp-1);
}

.nav-chevron {
  width: 16px; height: 16px;
  color: var(--text-4);
  transition: transform var(--t-smooth);
  flex-shrink: 0;
}
.nav-category-btn.open .nav-chevron { transform: rotate(90deg); }

/* Category icon colors */
.nav-cat-icon.cat-data       { background: var(--cat-data); }
.nav-cat-icon.cat-prep       { background: var(--cat-prep); }
.nav-cat-icon.cat-eda        { background: var(--cat-eda); }
.nav-cat-icon.cat-ml         { background: var(--cat-ml); }
.nav-cat-icon.cat-dl         { background: var(--cat-dl); }
.nav-cat-icon.cat-opt        { background: var(--cat-opt); }
.nav-cat-icon.cat-mlops      { background: var(--cat-mlops); }

.nav-subtopics {
  overflow: hidden;
  max-height: 0;
  transition: max-height 0.35s cubic-bezier(0.4,0,0.2,1);
  padding-left: calc(28px + var(--sp-3) + var(--sp-3));
}
.nav-subtopics.open { max-height: 800px; }

.nav-sublink {
  display: block;
  padding: 5px var(--sp-3);
  border-left: 2px solid var(--border);
  margin-left: -1px;
  color: var(--text-3);
  font-size: 0.825rem;
  transition: all var(--t-fast);
  border-radius: 0 var(--r-sm) var(--r-sm) 0;
}
.nav-sublink:hover { color: var(--primary); border-left-color: var(--primary); background: var(--primary-glow); }
.nav-sublink.active { color: var(--primary); border-left-color: var(--primary); font-weight: 600; }

/* ============================================
   Main Content Area
   ============================================ */
.main-content {
  flex: 1;
  margin-left: var(--sidebar-w);
  padding: var(--sp-8);
  min-width: 0;
}

/* ============================================
   HOME PAGE
   ============================================ */

/* Hero */
.hero-section {
  position: relative;
  background: var(--gradient-hero);
  border-radius: var(--r-xl);
  padding: var(--sp-20) var(--sp-8);
  margin-bottom: var(--sp-12);
  text-align: center;
  overflow: hidden;
  isolation: isolate;
}

/* Animated grid background */
.hero-section::before {
  content: '';
  position: absolute;
  inset: 0;
  background-image:
    linear-gradient(rgba(99,102,241,0.08) 1px, transparent 1px),
    linear-gradient(90deg, rgba(99,102,241,0.08) 1px, transparent 1px);
  background-size: 40px 40px;
  z-index: 0;
}

/* Radial glow blob */
.hero-section::after {
  content: '';
  position: absolute;
  top: -30%;
  left: 50%;
  transform: translateX(-50%);
  width: 600px;
  height: 400px;
  background: radial-gradient(ellipse, rgba(99,102,241,0.25) 0%, rgba(139,92,246,0.15) 40%, transparent 70%);
  z-index: 0;
  pointer-events: none;
}

.hero-content { position: relative; z-index: 1; max-width: 760px; margin: 0 auto; }

.hero-eyebrow {
  display: inline-flex;
  align-items: center;
  gap: var(--sp-2);
  background: rgba(99,102,241,0.2);
  border: 1px solid rgba(99,102,241,0.4);
  color: var(--primary-light);
  padding: var(--sp-1) var(--sp-4);
  border-radius: var(--r-full);
  font-size: 0.8rem;
  font-weight: 600;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  margin-bottom: var(--sp-6);
}
.hero-eyebrow-dot {
  width: 6px; height: 6px;
  background: var(--primary-light);
  border-radius: 50%;
  animation: pulse-dot 2s ease-in-out infinite;
}
@keyframes pulse-dot {
  0%, 100% { opacity: 1; transform: scale(1); }
  50% { opacity: 0.5; transform: scale(0.7); }
}

.hero-title {
  font-size: clamp(2.4rem, 5vw, 4rem);
  font-weight: 800;
  line-height: 1.1;
  letter-spacing: -0.03em;
  color: #fff;
  margin-bottom: var(--sp-5);
}
.hero-title .gradient-word {
  background: linear-gradient(135deg, #a5b4fc 0%, #c4b5fd 50%, #67e8f9 100%);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

.hero-subtitle {
  font-size: 1.15rem;
  color: rgba(255,255,255,0.65);
  max-width: 540px;
  margin: 0 auto var(--sp-8);
  line-height: 1.7;
}

.hero-cta {
  display: flex;
  gap: var(--sp-3);
  justify-content: center;
  flex-wrap: wrap;
  margin-bottom: var(--sp-10);
}

.btn-primary {
  display: inline-flex;
  align-items: center;
  gap: var(--sp-2);
  padding: var(--sp-3) var(--sp-6);
  background: var(--gradient-primary);
  color: #fff;
  font-weight: 600;
  font-size: 0.95rem;
  border-radius: var(--r-full);
  border: none;
  box-shadow: 0 4px 20px rgba(99,102,241,0.4);
  transition: transform var(--t-bounce), box-shadow var(--t-smooth);
}
.btn-primary:hover {
  transform: translateY(-2px);
  box-shadow: 0 8px 30px rgba(99,102,241,0.55);
}
.btn-primary svg { width: 16px; height: 16px; }

.btn-secondary {
  display: inline-flex;
  align-items: center;
  gap: var(--sp-2);
  padding: var(--sp-3) var(--sp-6);
  background: rgba(255,255,255,0.1);
  color: rgba(255,255,255,0.85);
  font-weight: 600;
  font-size: 0.95rem;
  border-radius: var(--r-full);
  border: 1px solid rgba(255,255,255,0.2);
  transition: background var(--t-fast), color var(--t-fast);
}
.btn-secondary:hover { background: rgba(255,255,255,0.18); color: #fff; }
.btn-secondary svg { width: 16px; height: 16px; }

/* Stats bar */
.hero-stats {
  display: flex;
  gap: var(--sp-8);
  justify-content: center;
  flex-wrap: wrap;
  padding-top: var(--sp-8);
  border-top: 1px solid rgba(255,255,255,0.1);
}
.stat-item { text-align: center; }
.stat-number {
  display: block;
  font-size: 2rem;
  font-weight: 800;
  background: linear-gradient(135deg, #a5b4fc, #67e8f9);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
  line-height: 1;
}
.stat-label {
  font-size: 0.78rem;
  color: rgba(255,255,255,0.5);
  text-transform: uppercase;
  letter-spacing: 0.06em;
  margin-top: var(--sp-1);
  display: block;
}

/* Section header */
.section-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-bottom: var(--sp-6);
}
.section-title {
  font-size: 1.5rem;
  font-weight: 700;
  color: var(--text-1);
}
.section-sub {
  font-size: 0.875rem;
  color: var(--text-3);
  margin-top: var(--sp-1);
}

/* Category Grid */
.categories-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
  gap: var(--sp-4);
  margin-bottom: var(--sp-12);
}

.category-card {
  position: relative;
  display: flex;
  flex-direction: column;
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r-lg);
  padding: var(--sp-6);
  text-decoration: none;
  color: var(--text-1);
  transition: transform var(--t-bounce), box-shadow var(--t-smooth), border-color var(--t-fast);
  overflow: hidden;
  animation: fadeInUp 0.5s both;
}
.category-card:hover {
  transform: translateY(-5px);
  box-shadow: var(--sh-lg);
}

/* Subtle colored top border per category */
.category-card::before {
  content: '';
  position: absolute;
  top: 0; left: 0; right: 0;
  height: 3px;
  border-radius: var(--r-lg) var(--r-lg) 0 0;
  background: var(--cat-color, var(--gradient-primary));
}

/* Faint background blob */
.category-card::after {
  content: '';
  position: absolute;
  top: -20px; right: -20px;
  width: 100px; height: 100px;
  border-radius: 50%;
  background: var(--cat-bg-color, rgba(99,102,241,0.08));
  pointer-events: none;
  transition: transform var(--t-smooth);
}
.category-card:hover::after { transform: scale(1.4); }

.cat-card-icon {
  width: 48px; height: 48px;
  border-radius: var(--r);
  display: flex; align-items: center; justify-content: center;
  margin-bottom: var(--sp-4);
  background: var(--cat-bg-color, rgba(99,102,241,0.12));
  color: var(--cat-color, var(--primary));
}
.cat-card-icon svg { width: 24px; height: 24px; }

.cat-card-name {
  font-size: 1.05rem;
  font-weight: 700;
  margin-bottom: var(--sp-2);
}
.cat-card-desc {
  font-size: 0.85rem;
  color: var(--text-3);
  line-height: 1.55;
  flex: 1;
  margin-bottom: var(--sp-4);
}

.cat-card-footer {
  display: flex;
  align-items: center;
  justify-content: space-between;
}
.cat-card-count {
  font-size: 0.75rem;
  font-weight: 600;
  color: var(--cat-color, var(--primary));
  background: var(--cat-bg-color, rgba(99,102,241,0.1));
  padding: 2px 10px;
  border-radius: var(--r-full);
}
.cat-card-arrow {
  width: 28px; height: 28px;
  border-radius: 50%;
  background: var(--surface-3);
  color: var(--text-3);
  display: flex; align-items: center; justify-content: center;
  transition: background var(--t-fast), color var(--t-fast), transform var(--t-fast);
}
.category-card:hover .cat-card-arrow {
  background: var(--cat-color, var(--primary));
  color: #fff;
  transform: translateX(3px);
}
.cat-card-arrow svg { width: 14px; height: 14px; }

/* Category color assignments (applied via class on template) */
.cat-data    { --cat-color: var(--cat-data);   --cat-bg-color: var(--cat-data-bg); }
.cat-prep    { --cat-color: var(--cat-prep);   --cat-bg-color: var(--cat-prep-bg); }
.cat-eda     { --cat-color: var(--cat-eda);    --cat-bg-color: var(--cat-eda-bg); }
.cat-ml      { --cat-color: var(--cat-ml);     --cat-bg-color: var(--cat-ml-bg); }
.cat-dl      { --cat-color: var(--cat-dl);     --cat-bg-color: var(--cat-dl-bg); }
.cat-opt     { --cat-color: var(--cat-opt);    --cat-bg-color: var(--cat-opt-bg); }
.cat-mlops   { --cat-color: var(--cat-mlops);  --cat-bg-color: var(--cat-mlops-bg); }

/* Featured Topics */
.featured-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
  gap: var(--sp-4);
  margin-bottom: var(--sp-12);
}
.featured-card {
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r-lg);
  padding: var(--sp-5);
  text-decoration: none;
  color: var(--text-1);
  display: flex;
  flex-direction: column;
  gap: var(--sp-3);
  transition: transform var(--t-bounce), box-shadow var(--t-smooth);
  animation: fadeInUp 0.5s both;
}
.featured-card:hover { transform: translateY(-3px); box-shadow: var(--sh-md); }

.featured-card-meta {
  display: flex;
  gap: var(--sp-2);
  align-items: center;
}
.featured-card-cat {
  font-size: 0.7rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  padding: 2px 8px;
  border-radius: var(--r-full);
  background: var(--cat-bg-color, rgba(99,102,241,0.12));
  color: var(--cat-color, var(--primary));
}
.featured-card-title {
  font-size: 1rem;
  font-weight: 700;
  line-height: 1.35;
  color: var(--text-1);
}
.featured-card-excerpt {
  font-size: 0.825rem;
  color: var(--text-3);
  line-height: 1.55;
  flex: 1;
}

/* How It Works */
.how-it-works {
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r-xl);
  padding: var(--sp-10) var(--sp-8);
  margin-bottom: var(--sp-12);
  text-align: center;
}
.how-it-works .section-title { margin-bottom: var(--sp-2); }
.how-it-works .section-sub { margin-bottom: var(--sp-10); }

.steps-grid {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: var(--sp-4);
  position: relative;
}

/* Connecting line */
.steps-grid::before {
  content: '';
  position: absolute;
  top: 24px;
  left: 12.5%;
  right: 12.5%;
  height: 2px;
  background: linear-gradient(90deg, var(--primary), var(--secondary), var(--accent));
  opacity: 0.3;
  z-index: 0;
}

.step-item {
  position: relative;
  z-index: 1;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: var(--sp-3);
  animation: fadeInUp 0.5s both;
}

.step-num {
  width: 48px; height: 48px;
  border-radius: 50%;
  background: var(--gradient-primary);
  color: #fff;
  font-weight: 800;
  font-size: 1.1rem;
  display: flex; align-items: center; justify-content: center;
  box-shadow: 0 4px 16px rgba(99,102,241,0.35);
  flex-shrink: 0;
}
.step-title { font-weight: 700; font-size: 1rem; color: var(--text-1); }
.step-desc { font-size: 0.825rem; color: var(--text-3); line-height: 1.5; }

/* ============================================
   CATEGORY PAGE
   ============================================ */
.cat-hero {
  border-radius: var(--r-xl);
  padding: var(--sp-10) var(--sp-8);
  margin-bottom: var(--sp-8);
  display: flex;
  align-items: center;
  gap: var(--sp-6);
  position: relative;
  overflow: hidden;
}
.cat-hero::before {
  content: '';
  position: absolute;
  inset: 0;
  background-image:
    linear-gradient(rgba(255,255,255,0.05) 1px, transparent 1px),
    linear-gradient(90deg, rgba(255,255,255,0.05) 1px, transparent 1px);
  background-size: 32px 32px;
}
.cat-hero-icon {
  width: 72px; height: 72px;
  background: rgba(255,255,255,0.2);
  border-radius: var(--r-lg);
  display: flex; align-items: center; justify-content: center;
  flex-shrink: 0;
  position: relative;
}
.cat-hero-icon svg { width: 36px; height: 36px; color: #fff; }
.cat-hero-text { position: relative; }
.cat-hero h1 { font-size: 2.2rem; font-weight: 800; color: #fff; margin-bottom: var(--sp-2); }
.cat-hero-desc { color: rgba(255,255,255,0.75); font-size: 1rem; max-width: 500px; }
.cat-hero-badge {
  margin-left: auto;
  position: relative;
  background: rgba(255,255,255,0.15);
  border: 1px solid rgba(255,255,255,0.25);
  color: #fff;
  padding: var(--sp-3) var(--sp-5);
  border-radius: var(--r-lg);
  text-align: center;
  flex-shrink: 0;
}
.cat-hero-badge-num { display: block; font-size: 2rem; font-weight: 800; }
.cat-hero-badge-label { font-size: 0.75rem; opacity: 0.8; }

/* Category hero colored backgrounds */
.cat-bg-data  { background: linear-gradient(135deg, #0891b2 0%, #0e7490 100%); }
.cat-bg-prep  { background: linear-gradient(135deg, #059669 0%, #047857 100%); }
.cat-bg-eda   { background: linear-gradient(135deg, #d97706 0%, #b45309 100%); }
.cat-bg-ml    { background: linear-gradient(135deg, #4f46e5 0%, #4338ca 100%); }
.cat-bg-dl    { background: linear-gradient(135deg, #7c3aed 0%, #6d28d9 100%); }
.cat-bg-opt   { background: linear-gradient(135deg, #e11d48 0%, #be123c 100%); }
.cat-bg-mlops { background: linear-gradient(135deg, #0d9488 0%, #0f766e 100%); }

/* Difficulty filter */
.difficulty-filter {
  display: flex;
  gap: var(--sp-2);
  margin-bottom: var(--sp-6);
  flex-wrap: wrap;
}
.filter-btn {
  padding: var(--sp-2) var(--sp-4);
  border: 1px solid var(--border);
  border-radius: var(--r-full);
  background: var(--surface-1);
  color: var(--text-3);
  font-size: 0.85rem;
  font-weight: 500;
  transition: all var(--t-fast);
}
.filter-btn:hover { border-color: var(--primary); color: var(--primary); }
.filter-btn.active {
  background: var(--gradient-primary);
  color: #fff;
  border-color: transparent;
  box-shadow: 0 2px 12px rgba(99,102,241,0.3);
}

/* Rich topic cards on category page */
.topics-list { display: flex; flex-direction: column; gap: var(--sp-3); }

.topic-card {
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r-lg);
  padding: var(--sp-5) var(--sp-6);
  text-decoration: none;
  color: var(--text-1);
  display: flex;
  flex-direction: column;
  gap: var(--sp-2);
  transition: all var(--t-smooth);
  position: relative;
  animation: fadeInUp 0.4s both;
}
.topic-card:hover {
  border-color: var(--primary);
  box-shadow: var(--sh-md), 0 0 0 1px var(--primary);
  transform: translateX(3px);
}
.topic-card::before {
  content: '';
  position: absolute;
  left: 0; top: 25%; bottom: 25%;
  width: 3px;
  background: var(--gradient-primary);
  border-radius: 0 var(--r-full) var(--r-full) 0;
  opacity: 0;
  transition: opacity var(--t-fast);
}
.topic-card:hover::before { opacity: 1; }

.topic-top-row {
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: var(--sp-4);
}
.topic-card h3 { font-size: 1.05rem; font-weight: 700; color: var(--text-1); flex: 1; }
.topic-excerpt { font-size: 0.85rem; color: var(--text-3); line-height: 1.55; }

.topic-bottom-row {
  display: flex;
  align-items: center;
  gap: var(--sp-3);
  flex-wrap: wrap;
}

/* Badges */
.difficulty-badge {
  font-size: 0.7rem;
  font-weight: 700;
  padding: 2px 10px;
  border-radius: var(--r-full);
  text-transform: uppercase;
  letter-spacing: 0.04em;
  flex-shrink: 0;
}
.difficulty-beginner     { background: rgba(34,197,94,0.12);  color: var(--diff-beginner); }
.difficulty-intermediate { background: rgba(245,158,11,0.12); color: var(--diff-intermediate); }
.difficulty-advanced     { background: rgba(244,63,94,0.12);  color: var(--diff-advanced); }

.category-badge {
  font-size: 0.7rem;
  font-weight: 600;
  padding: 2px 10px;
  border-radius: var(--r-full);
  background: var(--surface-3);
  color: var(--text-3);
  text-transform: uppercase;
  letter-spacing: 0.04em;
}
.read-time {
  font-size: 0.75rem;
  color: var(--text-4);
  display: flex;
  align-items: center;
  gap: 4px;
  margin-left: auto;
}
.read-time svg { width: 13px; height: 13px; }

/* ============================================
   TOPIC CONTENT PAGE
   ============================================ */
.topic-layout {
  display: grid;
  grid-template-columns: 1fr 240px;
  gap: var(--sp-8);
  align-items: start;
  max-width: 1100px;
}

.topic-content { min-width: 0; }

/* Breadcrumb */
.breadcrumb {
  display: flex;
  align-items: center;
  gap: var(--sp-2);
  font-size: 0.825rem;
  color: var(--text-4);
  margin-bottom: var(--sp-5);
  flex-wrap: wrap;
}
.breadcrumb a { color: var(--text-3); transition: color var(--t-fast); }
.breadcrumb a:hover { color: var(--primary); }
.breadcrumb-sep { color: var(--border-strong); }

/* Topic Header */
.topic-header-full { margin-bottom: var(--sp-8); }
.topic-header-full h1 {
  font-size: 2.25rem;
  font-weight: 800;
  line-height: 1.2;
  letter-spacing: -0.02em;
  color: var(--text-1);
  margin-bottom: var(--sp-4);
}
.topic-badges {
  display: flex;
  gap: var(--sp-2);
  align-items: center;
  flex-wrap: wrap;
}
.topic-meta-info {
  display: flex;
  gap: var(--sp-4);
  margin-top: var(--sp-3);
}
.topic-meta-item {
  display: flex;
  align-items: center;
  gap: 5px;
  font-size: 0.8rem;
  color: var(--text-4);
}
.topic-meta-item svg { width: 14px; height: 14px; }

/* Prerequisites Box */
.prerequisites-box {
  background: rgba(6,182,212,0.06);
  border: 1px solid rgba(6,182,212,0.25);
  border-radius: var(--r-lg);
  padding: var(--sp-4) var(--sp-5);
  margin-bottom: var(--sp-8);
  display: flex;
  align-items: flex-start;
  gap: var(--sp-3);
}
.prerequisites-box svg { width: 18px; height: 18px; color: var(--accent); flex-shrink: 0; margin-top: 1px; }
.prerequisites-box strong { color: var(--text-2); font-size: 0.875rem; }
.prereq-links { margin-top: var(--sp-1); display: flex; flex-wrap: wrap; gap: var(--sp-2); }
.prereq-link {
  color: var(--accent);
  font-size: 0.825rem;
  font-weight: 500;
  border-bottom: 1px dashed rgba(6,182,212,0.4);
  transition: border-color var(--t-fast);
}
.prereq-link:hover { border-bottom-color: var(--accent); }

/* Content Sections */
.content-section { margin-bottom: var(--sp-10); opacity: 0; transform: translateY(16px); }
.content-section.visible { opacity: 1; transform: none; transition: opacity 0.5s ease, transform 0.5s ease; }

.content-section h2 {
  font-size: 1.4rem;
  font-weight: 700;
  color: var(--text-1);
  margin-bottom: var(--sp-4);
  padding-bottom: var(--sp-3);
  border-bottom: 2px solid var(--border);
  display: flex;
  align-items: center;
  gap: var(--sp-2);
}
.content-section h2 .section-anchor {
  opacity: 0;
  color: var(--text-4);
  font-size: 1rem;
  transition: opacity var(--t-fast);
  margin-left: auto;
}
.content-section h2:hover .section-anchor { opacity: 1; }
.content-section h2 .section-anchor svg { width: 16px; height: 16px; }

/* Definition */
.definition p {
  font-size: 1.05rem;
  line-height: 1.75;
  color: var(--text-2);
}

/* Intuition */
.intuition-box {
  background: linear-gradient(135deg, rgba(139,92,246,0.06) 0%, rgba(6,182,212,0.06) 100%);
  border: 1px solid rgba(139,92,246,0.2);
  border-radius: var(--r-lg);
  padding: var(--sp-5);
  display: flex;
  gap: var(--sp-4);
  align-items: flex-start;
}
.intuition-icon { font-size: 1.5rem; flex-shrink: 0; margin-top: 2px; }
.intuition-box p { color: var(--text-2); line-height: 1.7; }

/* Formula */
.formula-box {
  background: var(--surface-2);
  border: 1px solid var(--border);
  border-radius: var(--r-lg);
  padding: var(--sp-6);
  margin-bottom: var(--sp-4);
  overflow-x: auto;
}
.math-label {
  font-size: 0.9rem;
  font-weight: 600;
  color: var(--text-2);
  margin-top: var(--sp-4);
  margin-bottom: var(--sp-1);
}
.math-label:first-child { margin-top: 0; }
.math-display { margin-bottom: var(--sp-3); overflow-x: auto; text-align: center; }
.math-prose {
  font-size: 0.825rem;
  color: var(--text-3);
  font-style: italic;
  margin-bottom: var(--sp-1);
  padding-left: var(--sp-3);
}
.formula-steps {
  background: var(--surface-2);
  border: 1px solid var(--border);
  border-radius: var(--r);
  padding: var(--sp-5);
}
.formula-steps h4 { margin-bottom: var(--sp-3); color: var(--text-2); font-size: 0.9rem; }
.formula-steps ol { padding-left: var(--sp-5); }
.formula-steps li { margin-bottom: var(--sp-2); color: var(--text-2); font-size: 0.9rem; }

/* Visualization Demo */
.viz-demo {
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r-lg);
  overflow: hidden;
}
.viz-demo-header {
  padding: var(--sp-3) var(--sp-5);
  background: var(--surface-3);
  border-bottom: 1px solid var(--border);
  display: flex;
  align-items: center;
  gap: var(--sp-3);
  font-size: 0.875rem;
  font-weight: 600;
  color: var(--text-2);
}
.viz-demo-header svg { width: 16px; height: 16px; color: var(--primary); }
.viz-demo-badge {
  font-size: 0.7rem;
  background: var(--primary-glow);
  color: var(--primary);
  padding: 1px 8px;
  border-radius: var(--r-full);
  font-weight: 600;
  margin-left: auto;
}
.viz-demo-body { padding: var(--sp-5); }
.viz-demo canvas { max-width: 100%; height: 280px !important; }

/* Use Cases */
.use-cases-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(230px, 1fr));
  gap: var(--sp-4);
}
.use-case-card {
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r-lg);
  padding: var(--sp-5);
  transition: box-shadow var(--t-fast);
}
.use-case-card:hover { box-shadow: var(--sh-md); }
.use-case-domain {
  display: inline-block;
  font-size: 0.7rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--primary);
  background: var(--primary-glow);
  padding: 2px 8px;
  border-radius: var(--r-full);
  margin-bottom: var(--sp-3);
}
.use-case-card p { color: var(--text-3); font-size: 0.875rem; line-height: 1.55; }

/* Code Section */
.code-tabs { display: flex; flex-direction: column; gap: var(--sp-4); }
.code-tab {
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r-lg);
  overflow: hidden;
}
.code-tab h4 {
  background: var(--surface-3);
  padding: var(--sp-3) var(--sp-5);
  font-size: 0.825rem;
  font-weight: 600;
  color: var(--text-2);
  border-bottom: 1px solid var(--border);
}
.code-explanation {
  padding: var(--sp-3) var(--sp-5);
  font-size: 0.85rem;
  color: var(--text-3);
  border-bottom: 1px solid var(--border);
  line-height: 1.55;
}
.code-wrapper { position: relative; }
.code-tab pre { background: var(--code-bg); padding: var(--sp-5); overflow-x: auto; }
.code-tab code {
  font-family: var(--font-mono);
  font-size: 0.85rem;
  line-height: 1.65;
  color: var(--code-text);
  white-space: pre;
}
.copy-btn {
  position: absolute;
  top: 0.6rem; right: 0.75rem;
  z-index: 10;
  background: rgba(255,255,255,0.1);
  color: #94a3b8;
  border: 1px solid rgba(255,255,255,0.15);
  border-radius: var(--r-sm);
  padding: 3px 10px;
  font-size: 0.75rem;
  font-family: var(--font-sans);
  font-weight: 500;
  cursor: pointer;
  backdrop-filter: blur(4px);
  transition: background var(--t-fast), color var(--t-fast), border-color var(--t-fast);
}
.copy-btn:hover { background: rgba(255,255,255,0.18); color: #e2e8f0; border-color: rgba(255,255,255,0.3); }
.copy-btn.copied { background: rgba(34,197,94,0.25); color: #4ade80; border-color: rgba(34,197,94,0.4); }

/* Guidance */
.guidance-box {
  border-radius: var(--r-lg);
  padding: var(--sp-5);
  margin-bottom: var(--sp-4);
}
.guidance-box.good { background: rgba(34,197,94,0.06); border: 1px solid rgba(34,197,94,0.2); }
.guidance-box.bad  { background: rgba(244,63,94,0.06);  border: 1px solid rgba(244,63,94,0.2); }
.guidance-box h4 { margin-bottom: var(--sp-3); font-size: 0.9rem; }
.guidance-box ul { padding-left: var(--sp-5); }
.guidance-box li { margin-bottom: var(--sp-2); color: var(--text-2); font-size: 0.9rem; }

/* Pitfalls */
.pitfalls-box {
  background: rgba(245,158,11,0.06);
  border: 1px solid rgba(245,158,11,0.25);
  border-radius: var(--r-lg);
  padding: var(--sp-5);
}
.pitfalls-box ul { list-style: none; padding: 0; }
.pitfalls-box li {
  display: flex;
  gap: var(--sp-3);
  margin-bottom: var(--sp-3);
  color: var(--text-2);
  font-size: 0.9rem;
  align-items: flex-start;
}
.pitfall-icon { color: #f59e0b; flex-shrink: 0; margin-top: 2px; }
.pitfall-icon svg { width: 16px; height: 16px; }

/* Related Topics */
.related-topics { display: flex; flex-wrap: wrap; gap: var(--sp-2); }
.related-topic-link {
  background: var(--surface-2);
  border: 1px solid var(--border);
  color: var(--text-2);
  text-decoration: none;
  padding: var(--sp-2) var(--sp-4);
  border-radius: var(--r-full);
  font-size: 0.85rem;
  font-weight: 500;
  transition: all var(--t-fast);
}
.related-topic-link:hover {
  background: var(--gradient-primary);
  color: #fff;
  border-color: transparent;
  transform: translateY(-1px);
}

/* Prev/Next Navigation */
.prev-next-nav {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--sp-4);
  margin-top: var(--sp-10);
  padding-top: var(--sp-8);
  border-top: 1px solid var(--border);
}
.prev-next-btn {
  display: flex;
  flex-direction: column;
  padding: var(--sp-5);
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r-lg);
  text-decoration: none;
  color: var(--text-1);
  transition: all var(--t-fast);
  gap: var(--sp-2);
}
.prev-next-btn:hover { border-color: var(--primary); box-shadow: var(--sh-md); }
.prev-next-btn.next { text-align: right; }
.prev-next-label {
  font-size: 0.75rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--text-4);
  display: flex;
  align-items: center;
  gap: var(--sp-2);
}
.prev-next-btn.next .prev-next-label { justify-content: flex-end; }
.prev-next-label svg { width: 14px; height: 14px; }
.prev-next-title { font-size: 0.95rem; font-weight: 700; color: var(--text-1); }

/* Table of Contents (sticky sidebar on topic page) */
.toc-sidebar {
  position: sticky;
  top: calc(var(--header-h) + var(--sp-8));
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r-lg);
  padding: var(--sp-4);
  font-size: 0.82rem;
  max-height: calc(100vh - var(--header-h) - var(--sp-16));
  overflow-y: auto;
  scrollbar-width: thin;
}
.toc-title {
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--text-4);
  margin-bottom: var(--sp-3);
}
.toc-list { list-style: none; padding: 0; display: flex; flex-direction: column; gap: 2px; }
.toc-link {
  display: block;
  padding: 4px var(--sp-3);
  border-left: 2px solid var(--border);
  color: var(--text-3);
  transition: all var(--t-fast);
  border-radius: 0 var(--r-sm) var(--r-sm) 0;
  line-height: 1.4;
}
.toc-link:hover { color: var(--primary); border-left-color: var(--primary); background: var(--primary-glow); }
.toc-link.active { color: var(--primary); border-left-color: var(--primary); font-weight: 600; }

/* ============================================
   SEARCH PAGE
   ============================================ */
.search-hero {
  text-align: center;
  padding: var(--sp-16) 0 var(--sp-10);
}
.search-hero h1 { font-size: 2.5rem; font-weight: 800; color: var(--text-1); margin-bottom: var(--sp-4); }
.search-hero p { color: var(--text-3); font-size: 1rem; margin-bottom: var(--sp-6); }
.search-hero-form {
  max-width: 520px;
  margin: 0 auto;
  display: flex;
  box-shadow: var(--sh-lg);
  border-radius: var(--r-full);
  overflow: hidden;
}
.search-hero-input {
  flex: 1;
  padding: var(--sp-4) var(--sp-6);
  border: 1px solid var(--border);
  border-right: none;
  border-radius: var(--r-full) 0 0 var(--r-full);
  font-size: 1rem;
  outline: none;
  background: var(--surface-1);
  color: var(--text-1);
  transition: border-color var(--t-fast);
}
.search-hero-input:focus { border-color: var(--primary); }
.search-hero-btn {
  padding: var(--sp-4) var(--sp-6);
  background: var(--gradient-primary);
  color: #fff;
  border: none;
  font-size: 0.95rem;
  font-weight: 600;
  border-radius: 0 var(--r-full) var(--r-full) 0;
  transition: opacity var(--t-fast);
}
.search-hero-btn:hover { opacity: 0.88; }

.search-results-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-bottom: var(--sp-5);
}
.search-results-count { font-size: 0.9rem; color: var(--text-3); }
.search-results-count strong { color: var(--text-1); }

.search-result-card {
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r-lg);
  padding: var(--sp-5) var(--sp-6);
  text-decoration: none;
  color: var(--text-1);
  display: block;
  margin-bottom: var(--sp-3);
  transition: all var(--t-smooth);
}
.search-result-card:hover { border-color: var(--primary); box-shadow: var(--sh-md); transform: translateX(3px); }
.search-result-row { display: flex; align-items: flex-start; gap: var(--sp-4); flex-wrap: wrap; }
.search-result-title { font-size: 1rem; font-weight: 700; margin-bottom: var(--sp-2); }
.search-result-title mark { background: rgba(99,102,241,0.18); color: var(--primary); border-radius: 3px; padding: 0 2px; }
.search-result-excerpt { font-size: 0.85rem; color: var(--text-3); line-height: 1.55; }

.search-empty {
  text-align: center;
  padding: var(--sp-16) 0;
}
.search-empty-icon { font-size: 3rem; margin-bottom: var(--sp-4); opacity: 0.4; }
.search-empty h3 { font-size: 1.25rem; font-weight: 700; color: var(--text-2); margin-bottom: var(--sp-2); }
.search-empty p { color: var(--text-3); margin-bottom: var(--sp-6); }

.search-suggestions { display: flex; flex-wrap: wrap; gap: var(--sp-2); justify-content: center; }
.suggestion-chip {
  padding: var(--sp-2) var(--sp-4);
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r-full);
  font-size: 0.85rem;
  color: var(--text-2);
  font-weight: 500;
  text-decoration: none;
  transition: all var(--t-fast);
}
.suggestion-chip:hover { border-color: var(--primary); color: var(--primary); background: var(--primary-glow); }

/* ============================================
   ABOUT PAGE
   ============================================ */
.about-hero {
  background: var(--gradient-hero);
  border-radius: var(--r-xl);
  padding: var(--sp-16) var(--sp-8);
  text-align: center;
  margin-bottom: var(--sp-10);
  position: relative;
  overflow: hidden;
}
.about-hero::before {
  content: '';
  position: absolute;
  inset: 0;
  background-image:
    linear-gradient(rgba(99,102,241,0.07) 1px, transparent 1px),
    linear-gradient(90deg, rgba(99,102,241,0.07) 1px, transparent 1px);
  background-size: 36px 36px;
}
.about-hero-content { position: relative; }
.about-hero h1 { font-size: 2.5rem; font-weight: 800; color: #fff; margin-bottom: var(--sp-3); }
.about-hero p { font-size: 1.05rem; color: rgba(255,255,255,0.7); max-width: 500px; margin: 0 auto; }

.about-stats {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(160px, 1fr));
  gap: var(--sp-4);
  margin-bottom: var(--sp-10);
}
.about-stat {
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r-lg);
  padding: var(--sp-5);
  text-align: center;
  transition: box-shadow var(--t-fast);
}
.about-stat:hover { box-shadow: var(--sh-md); }
.about-stat-num { font-size: 2.2rem; font-weight: 800; background: var(--gradient-primary); -webkit-background-clip: text; -webkit-text-fill-color: transparent; background-clip: text; display: block; }
.about-stat-label { font-size: 0.8rem; color: var(--text-3); margin-top: var(--sp-1); }

.about-section { margin-bottom: var(--sp-10); }
.about-section h2 {
  font-size: 1.5rem; font-weight: 700; color: var(--text-1);
  margin-bottom: var(--sp-5);
  padding-bottom: var(--sp-3);
  border-bottom: 2px solid var(--border);
}
.about-section p { color: var(--text-2); line-height: 1.75; font-size: 0.95rem; }

.persona-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(220px, 1fr));
  gap: var(--sp-4);
}
.persona-card {
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r-lg);
  padding: var(--sp-6);
  transition: box-shadow var(--t-fast);
}
.persona-card:hover { box-shadow: var(--sh-md); }
.persona-icon {
  width: 52px; height: 52px;
  border-radius: var(--r);
  background: var(--gradient-primary);
  display: flex; align-items: center; justify-content: center;
  margin-bottom: var(--sp-4);
  font-size: 1.4rem;
}
.persona-title { font-size: 1rem; font-weight: 700; margin-bottom: var(--sp-2); }
.persona-desc { font-size: 0.85rem; color: var(--text-3); line-height: 1.55; }

.philosophy-grid {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: var(--sp-4);
}
.philosophy-card {
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r-lg);
  padding: var(--sp-5);
  display: flex;
  gap: var(--sp-4);
  transition: box-shadow var(--t-fast);
}
.philosophy-card:hover { box-shadow: var(--sh-md); }
.philosophy-num {
  width: 36px; height: 36px;
  border-radius: 50%;
  background: var(--gradient-primary);
  color: #fff;
  font-weight: 800;
  font-size: 0.9rem;
  display: flex; align-items: center; justify-content: center;
  flex-shrink: 0;
}
.philosophy-card h4 { font-weight: 700; font-size: 0.95rem; margin-bottom: var(--sp-1); }
.philosophy-card p { font-size: 0.85rem; color: var(--text-3); line-height: 1.5; }

.about-cta {
  background: var(--gradient-primary);
  border-radius: var(--r-xl);
  padding: var(--sp-10) var(--sp-8);
  text-align: center;
  margin-bottom: var(--sp-8);
}
.about-cta h2 { color: #fff; font-size: 1.75rem; font-weight: 800; margin-bottom: var(--sp-3); }
.about-cta p { color: rgba(255,255,255,0.75); margin-bottom: var(--sp-6); }
.about-cta-btn {
  display: inline-flex;
  align-items: center;
  gap: var(--sp-2);
  padding: var(--sp-3) var(--sp-8);
  background: #fff;
  color: var(--primary);
  font-weight: 700;
  font-size: 0.95rem;
  border-radius: var(--r-full);
  border: none;
  box-shadow: var(--sh-lg);
  transition: transform var(--t-bounce), box-shadow var(--t-smooth);
  text-decoration: none;
}
.about-cta-btn:hover { transform: translateY(-2px); box-shadow: var(--sh-xl); }

/* ============================================
   404 PAGE
   ============================================ */
.error-page { text-align: center; padding: var(--sp-16) 0; }
.error-visual {
  width: 160px; height: 160px;
  margin: 0 auto var(--sp-8);
  position: relative;
}
.error-visual-ring {
  position: absolute;
  border-radius: 50%;
  animation: rotate-slow 8s linear infinite;
}
.error-visual-ring-1 {
  inset: 0;
  border: 2px dashed rgba(99,102,241,0.25);
  animation-direction: normal;
}
.error-visual-ring-2 {
  inset: 20px;
  border: 2px dashed rgba(139,92,246,0.3);
  animation-direction: reverse;
  animation-duration: 6s;
}
.error-num {
  position: absolute;
  inset: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 3rem;
  font-weight: 900;
  background: var(--gradient-primary);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}
@keyframes rotate-slow { from { transform: rotate(0deg); } to { transform: rotate(360deg); } }

.error-page h1 { font-size: 1.75rem; font-weight: 800; color: var(--text-1); margin-bottom: var(--sp-3); }
.error-page p { color: var(--text-3); margin-bottom: var(--sp-8); font-size: 1rem; }
.error-page .topics-preview {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(180px, 1fr));
  gap: var(--sp-3);
  margin-top: var(--sp-8);
  max-width: 640px;
  margin-left: auto;
  margin-right: auto;
}
.error-topic-chip {
  padding: var(--sp-3) var(--sp-4);
  background: var(--surface-1);
  border: 1px solid var(--border);
  border-radius: var(--r-lg);
  text-decoration: none;
  color: var(--text-2);
  font-size: 0.875rem;
  font-weight: 500;
  transition: all var(--t-fast);
  display: flex;
  align-items: center;
  gap: var(--sp-2);
}
.error-topic-chip:hover { border-color: var(--primary); color: var(--primary); background: var(--primary-glow); }
.error-topic-chip svg { width: 14px; height: 14px; }
.error-back-btn {
  display: inline-flex;
  align-items: center;
  gap: var(--sp-2);
  padding: var(--sp-3) var(--sp-6);
  background: var(--gradient-primary);
  color: #fff;
  font-weight: 600;
  font-size: 0.95rem;
  border-radius: var(--r-full);
  text-decoration: none;
  box-shadow: 0 4px 20px rgba(99,102,241,0.35);
  transition: transform var(--t-bounce);
}
.error-back-btn:hover { transform: translateY(-2px); }
.error-back-btn svg { width: 16px; height: 16px; }

/* ============================================
   FOOTER
   ============================================ */
.footer {
  margin-left: var(--sidebar-w);
  padding: var(--sp-8);
  border-top: 1px solid var(--border);
  background: var(--surface-1);
}
.footer-inner {
  max-width: var(--content-max);
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: var(--sp-4);
  flex-wrap: wrap;
}
.footer-logo {
  display: flex;
  align-items: center;
  gap: var(--sp-2);
  font-weight: 700;
  font-size: 0.9rem;
  color: var(--text-1);
  text-decoration: none;
}
.footer-logo-icon {
  width: 24px; height: 24px;
  background: var(--gradient-primary);
  border-radius: var(--r-sm);
  display: flex; align-items: center; justify-content: center;
}
.footer-logo-icon svg { width: 12px; height: 12px; color: #fff; }
.footer-copy { font-size: 0.8rem; color: var(--text-4); }
.footer-links { display: flex; gap: var(--sp-5); }
.footer-links a { font-size: 0.8rem; color: var(--text-3); text-decoration: none; transition: color var(--t-fast); }
.footer-links a:hover { color: var(--primary); }

/* Mobile search overlay */
.search-overlay {
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,0.7);
  backdrop-filter: blur(8px);
  z-index: 300;
  display: flex;
  align-items: flex-start;
  justify-content: center;
  padding-top: 80px;
  opacity: 0;
  pointer-events: none;
  transition: opacity var(--t-smooth);
}
.search-overlay.open { opacity: 1; pointer-events: all; }
.search-overlay-box {
  width: 90%;
  max-width: 560px;
  display: flex;
  border-radius: var(--r-xl);
  overflow: hidden;
  box-shadow: var(--sh-xl);
}
.search-overlay-input {
  flex: 1;
  padding: var(--sp-4) var(--sp-6);
  border: none;
  font-size: 1.05rem;
  outline: none;
  background: var(--surface-1);
  color: var(--text-1);
}
.search-overlay-input::placeholder { color: var(--text-4); }
.search-overlay-btn {
  padding: var(--sp-4) var(--sp-5);
  background: var(--gradient-primary);
  border: none;
  color: #fff;
  font-size: 1rem;
  font-weight: 600;
}

/* ============================================
   Animations
   ============================================ */
@keyframes fadeInUp {
  from { opacity: 0; transform: translateY(20px); }
  to   { opacity: 1; transform: none; }
}
@keyframes fadeInScale {
  from { opacity: 0; transform: scale(0.95); }
  to   { opacity: 1; transform: scale(1); }
}
@keyframes shimmer {
  from { background-position: -200% center; }
  to   { background-position: 200% center; }
}

/* Staggered animation delays for grids */
.category-card:nth-child(1) { animation-delay: 0.05s; }
.category-card:nth-child(2) { animation-delay: 0.10s; }
.category-card:nth-child(3) { animation-delay: 0.15s; }
.category-card:nth-child(4) { animation-delay: 0.20s; }
.category-card:nth-child(5) { animation-delay: 0.25s; }
.category-card:nth-child(6) { animation-delay: 0.30s; }
.category-card:nth-child(7) { animation-delay: 0.35s; }

/* ============================================
   RESPONSIVE
   ============================================ */
@media (max-width: 1200px) {
  .topic-layout { grid-template-columns: 1fr; }
  .toc-sidebar { display: none; }
}

@media (max-width: 1024px) {
  .steps-grid { grid-template-columns: repeat(2, 1fr); }
  .steps-grid::before { display: none; }
  .philosophy-grid { grid-template-columns: 1fr; }
}

@media (max-width: 768px) {
  .sidebar {
    transform: translateX(-100%);
    transition: transform var(--t-smooth);
    z-index: 130;
  }
  .sidebar.open { transform: translateX(0); }
  .sidebar-backdrop {
    display: none;
    position: fixed;
    inset: 0;
    background: rgba(0,0,0,0.5);
    backdrop-filter: blur(4px);
    z-index: 125;
  }
  .sidebar-backdrop.open { display: block; }

  .main-content { margin-left: 0; padding: var(--sp-5); }
  .footer { margin-left: 0; }

  .nav-toggle { display: flex; }
  .search-form { display: none; }
  .search-icon-btn { display: flex; }

  .hero-section { padding: var(--sp-12) var(--sp-5); border-radius: var(--r-lg); }
  .hero-title { font-size: 2rem; }
  .hero-stats { gap: var(--sp-5); }
  .stat-number { font-size: 1.5rem; }

  .categories-grid { grid-template-columns: repeat(2, 1fr); }

  .steps-grid { grid-template-columns: 1fr 1fr; }

  .cat-hero { flex-direction: column; text-align: center; padding: var(--sp-8) var(--sp-5); }
  .cat-hero h1 { font-size: 1.75rem; }
  .cat-hero-badge { margin-left: 0; }

  .prev-next-nav { grid-template-columns: 1fr; }
  .prev-next-btn.next { text-align: left; }
  .prev-next-btn.next .prev-next-label { justify-content: flex-start; }

  .footer-inner { flex-direction: column; text-align: center; }

  .how-it-works { padding: var(--sp-8) var(--sp-5); }
}

@media (max-width: 480px) {
  :root { --sidebar-w: 280px; }
  .categories-grid { grid-template-columns: 1fr; }
  .steps-grid { grid-template-columns: 1fr; }
  .hero-cta { flex-direction: column; align-items: center; }
  .use-cases-grid { grid-template-columns: 1fr; }
  .featured-grid { grid-template-columns: 1fr; }
  .persona-grid { grid-template-columns: 1fr; }
  .about-stats { grid-template-columns: repeat(2, 1fr); }
}