@import url('https://fonts.googleapis.com/css2?family=Literata:ital,wght@0,300;0,400;0,500;0,600;0,700;1,400;1,500&family=Playfair+Display:wght@400;500;600;700;800&family=JetBrains+Mono:wght@300;400;500&display=swap');

:root {
  --bg-primary: #0f1118;
  --bg-secondary: #161822;
  --bg-card: #1c1f2e;
  --bg-card-hover: #222640;
  --bg-highlight: #1a2332;
  --text-primary: #e8e4dc;
  --text-secondary: #9a9690;
  --text-muted: #5c5955;
  --accent-teal: #4ecdc4;
  --accent-teal-dim: #2a7a74;
  --accent-amber: #f0a500;
  --accent-amber-dim: #7a5500;
  --accent-coral: #e07a5f;
  --accent-blue: #5b8def;
  --accent-purple: #a78bfa;
  --accent-green: #6bcb77;
  --border-subtle: #2a2d3a;
  --border-accent: #3a3d4a;
  --shadow-card: 0 4px 24px rgba(0,0,0,0.4);
  --shadow-glow: 0 0 30px rgba(78,205,196,0.08);
  --radius: 12px;
  --radius-sm: 8px;
  --max-width: 860px;
  --transition: 0.3s cubic-bezier(0.4, 0, 0.2, 1);
}

*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  scroll-behavior: smooth;
  font-size: 17px;
}

body {
  font-family: 'Literata', Georgia, serif;
  background: var(--bg-primary);
  color: var(--text-primary);
  line-height: 1.8;
  min-height: 100vh;
  -webkit-font-smoothing: antialiased;
}

/* ===== BACKGROUND TEXTURE ===== */
body::before {
  content: '';
  position: fixed;
  inset: 0;
  background:
    radial-gradient(ellipse at 20% 0%, rgba(78,205,196,0.04) 0%, transparent 50%),
    radial-gradient(ellipse at 80% 100%, rgba(240,165,0,0.03) 0%, transparent 50%);
  pointer-events: none;
  z-index: 0;
}

/* ===== SCROLLBAR ===== */
::-webkit-scrollbar { width: 6px; }
::-webkit-scrollbar-track { background: var(--bg-primary); }
::-webkit-scrollbar-thumb { background: var(--border-accent); border-radius: 3px; }
::-webkit-scrollbar-thumb:hover { background: var(--accent-teal-dim); }

/* ===== LAYOUT ===== */
.page-wrapper {
  position: relative;
  z-index: 1;
}

.top-nav {
  position: sticky;
  top: 0;
  z-index: 100;
  background: rgba(15,17,24,0.92);
  backdrop-filter: blur(16px);
  border-bottom: 1px solid var(--border-subtle);
  padding: 0.75rem 2rem;
  display: flex;
  align-items: center;
  gap: 1.5rem;
}

.top-nav a {
  color: var(--text-secondary);
  text-decoration: none;
  font-size: 0.82rem;
  font-weight: 500;
  letter-spacing: 0.03em;
  transition: color var(--transition);
  font-family: 'JetBrains Mono', monospace;
}

.top-nav a:hover,
.top-nav a.active {
  color: var(--accent-teal);
}

.top-nav .nav-brand {
  color: var(--text-primary);
  font-family: 'Playfair Display', serif;
  font-size: 1rem;
  font-weight: 700;
  margin-right: auto;
  letter-spacing: -0.02em;
}

.content {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 3rem 2rem 6rem;
}

/* ===== HERO (index page) ===== */
.hero {
  text-align: center;
  padding: 6rem 0 4rem;
}

.hero h1 {
  font-family: 'Playfair Display', serif;
  font-size: 3.2rem;
  font-weight: 800;
  line-height: 1.15;
  letter-spacing: -0.03em;
  margin-bottom: 1.2rem;
  background: linear-gradient(135deg, var(--text-primary) 0%, var(--accent-teal) 100%);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

.hero .subtitle {
  font-size: 1.1rem;
  color: var(--text-secondary);
  max-width: 540px;
  margin: 0 auto 2.5rem;
  line-height: 1.7;
}

.hero .tag {
  display: inline-block;
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.72rem;
  color: var(--accent-teal);
  border: 1px solid var(--accent-teal-dim);
  padding: 0.3rem 0.8rem;
  border-radius: 20px;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  margin-bottom: 2rem;
}

/* ===== CHAPTER CARDS ===== */
.chapter-grid {
  display: grid;
  gap: 1.5rem;
  margin-top: 2rem;
}

.chapter-card {
  display: block;
  background: var(--bg-card);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius);
  padding: 2rem 2.2rem;
  text-decoration: none;
  transition: all var(--transition);
  position: relative;
  overflow: hidden;
}

.chapter-card::before {
  content: '';
  position: absolute;
  left: 0;
  top: 0;
  bottom: 0;
  width: 3px;
  background: var(--card-accent, var(--accent-teal));
  opacity: 0;
  transition: opacity var(--transition);
}

.chapter-card:hover {
  background: var(--bg-card-hover);
  border-color: var(--border-accent);
  box-shadow: var(--shadow-card);
  transform: translateY(-2px);
}

.chapter-card:hover::before {
  opacity: 1;
}

.chapter-card .ch-number {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.7rem;
  color: var(--card-accent, var(--accent-teal));
  letter-spacing: 0.1em;
  text-transform: uppercase;
  margin-bottom: 0.6rem;
}

.chapter-card h3 {
  font-family: 'Playfair Display', serif;
  font-size: 1.4rem;
  color: var(--text-primary);
  margin-bottom: 0.6rem;
  font-weight: 600;
}

.chapter-card p {
  color: var(--text-secondary);
  font-size: 0.9rem;
  line-height: 1.6;
}

.chapter-card .topics {
  display: flex;
  flex-wrap: wrap;
  gap: 0.4rem;
  margin-top: 1rem;
}

.chapter-card .topic-tag {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.65rem;
  color: var(--text-muted);
  background: rgba(255,255,255,0.04);
  padding: 0.2rem 0.6rem;
  border-radius: 4px;
  letter-spacing: 0.04em;
}

/* ===== CHAPTER PAGE HEADINGS ===== */
.chapter-header {
  padding: 3rem 0 2rem;
  border-bottom: 1px solid var(--border-subtle);
  margin-bottom: 3rem;
}

.chapter-header .ch-label {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.75rem;
  color: var(--accent-teal);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  margin-bottom: 0.8rem;
}

.chapter-header h1 {
  font-family: 'Playfair Display', serif;
  font-size: 2.4rem;
  font-weight: 700;
  line-height: 1.2;
  letter-spacing: -0.02em;
}

.chapter-header .ch-desc {
  color: var(--text-secondary);
  margin-top: 0.8rem;
  font-size: 1rem;
}

/* ===== TABLE OF CONTENTS ===== */
.toc {
  background: var(--bg-card);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius);
  padding: 1.8rem 2rem;
  margin-bottom: 3rem;
}

.toc-title {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.72rem;
  color: var(--accent-teal);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  margin-bottom: 1rem;
}

.toc ol {
  list-style: none;
  counter-reset: toc-counter;
}

.toc ol li {
  counter-increment: toc-counter;
  margin-bottom: 0.4rem;
}

.toc ol li a {
  color: var(--text-secondary);
  text-decoration: none;
  font-size: 0.9rem;
  display: flex;
  align-items: baseline;
  gap: 0.6rem;
  padding: 0.25rem 0;
  transition: color var(--transition);
}

.toc ol li a::before {
  content: counter(toc-counter, decimal-leading-zero);
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.7rem;
  color: var(--text-muted);
  min-width: 1.4rem;
}

.toc ol li a:hover {
  color: var(--accent-teal);
}

/* ===== SECTION HEADINGS ===== */
.section {
  margin-bottom: 4rem;
  scroll-margin-top: 4rem;
}

.section h2 {
  font-family: 'Playfair Display', serif;
  font-size: 1.7rem;
  font-weight: 600;
  margin-bottom: 1.2rem;
  padding-bottom: 0.6rem;
  border-bottom: 1px solid var(--border-subtle);
  letter-spacing: -0.01em;
}

.section h3 {
  font-family: 'Playfair Display', serif;
  font-size: 1.25rem;
  font-weight: 600;
  margin-top: 2.5rem;
  margin-bottom: 0.8rem;
  color: var(--accent-teal);
}

.section h4 {
  font-size: 1rem;
  font-weight: 600;
  margin-top: 2rem;
  margin-bottom: 0.6rem;
  color: var(--accent-amber);
}

/* ===== TEXT ===== */
p {
  margin-bottom: 1rem;
}

strong {
  color: var(--text-primary);
  font-weight: 600;
}

em {
  color: var(--accent-teal);
  font-style: italic;
}

/* ===== DEFINITION / CONCEPT / WHY BOXES ===== */
.def-box, .concept-box, .why-box, .important-box, .example-box, .step-box {
  border-radius: var(--radius-sm);
  padding: 1.4rem 1.6rem;
  margin: 1.5rem 0;
  border-left: 3px solid;
}

.def-box {
  background: rgba(240,165,0,0.06);
  border-color: var(--accent-amber);
}

.def-box .box-label {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.68rem;
  color: var(--accent-amber);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  margin-bottom: 0.5rem;
}

.concept-box {
  background: rgba(78,205,196,0.06);
  border-color: var(--accent-teal);
}

.concept-box .box-label {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.68rem;
  color: var(--accent-teal);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  margin-bottom: 0.5rem;
}

.why-box {
  background: rgba(91,141,239,0.06);
  border-color: var(--accent-blue);
}

.why-box .box-label {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.68rem;
  color: var(--accent-blue);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  margin-bottom: 0.5rem;
}

.important-box {
  background: rgba(224,122,95,0.06);
  border-color: var(--accent-coral);
}

.important-box .box-label {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.68rem;
  color: var(--accent-coral);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  margin-bottom: 0.5rem;
}

.example-box {
  background: rgba(167,139,250,0.06);
  border-color: var(--accent-purple);
}

.example-box .box-label {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.68rem;
  color: var(--accent-purple);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  margin-bottom: 0.5rem;
}

.step-box {
  background: rgba(107,203,119,0.06);
  border-color: var(--accent-green);
}

.step-box .box-label {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.68rem;
  color: var(--accent-green);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  margin-bottom: 0.5rem;
}

/* ===== MATH ===== */
.math-block {
  background: var(--bg-secondary);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-sm);
  padding: 1.2rem 1.6rem;
  margin: 1.2rem 0;
  overflow-x: auto;
  text-align: center;
}

.math-block .MathJax {
  font-size: 1.05em !important;
}

/* ===== LISTS ===== */
ul, ol {
  padding-left: 1.4rem;
  margin-bottom: 1rem;
}

li {
  margin-bottom: 0.4rem;
}

li::marker {
  color: var(--accent-teal-dim);
}

/* ===== CODE ===== */
code {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.85em;
  background: rgba(255,255,255,0.06);
  padding: 0.15rem 0.4rem;
  border-radius: 4px;
  color: var(--accent-purple);
}

pre {
  background: var(--bg-secondary);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-sm);
  padding: 1.2rem 1.4rem;
  overflow-x: auto;
  margin: 1.2rem 0;
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.82rem;
  line-height: 1.7;
  color: var(--text-secondary);
}

/* ===== TABLES ===== */
table {
  width: 100%;
  border-collapse: collapse;
  margin: 1.5rem 0;
  font-size: 0.9rem;
}

th {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.72rem;
  color: var(--accent-teal);
  letter-spacing: 0.06em;
  text-transform: uppercase;
  text-align: left;
  padding: 0.8rem 1rem;
  border-bottom: 2px solid var(--border-accent);
  background: var(--bg-secondary);
}

td {
  padding: 0.7rem 1rem;
  border-bottom: 1px solid var(--border-subtle);
  color: var(--text-secondary);
}

tr:hover td {
  background: rgba(255,255,255,0.02);
}

/* ===== CHAPTER NAVIGATION (bottom) ===== */
.chapter-nav {
  display: flex;
  justify-content: space-between;
  gap: 1rem;
  margin-top: 4rem;
  padding-top: 2rem;
  border-top: 1px solid var(--border-subtle);
}

.chapter-nav a {
  display: flex;
  flex-direction: column;
  gap: 0.3rem;
  text-decoration: none;
  padding: 1rem 1.4rem;
  background: var(--bg-card);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-sm);
  transition: all var(--transition);
  max-width: 48%;
}

.chapter-nav a:hover {
  background: var(--bg-card-hover);
  border-color: var(--accent-teal-dim);
}

.chapter-nav a .nav-label {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.65rem;
  color: var(--accent-teal);
  letter-spacing: 0.1em;
  text-transform: uppercase;
}

.chapter-nav a .nav-title {
  font-family: 'Playfair Display', serif;
  color: var(--text-primary);
  font-size: 0.95rem;
}

.chapter-nav a.next {
  margin-left: auto;
  text-align: right;
}

/* ===== PROGRESS INDICATOR ===== */
.progress-bar {
  position: fixed;
  top: 0;
  left: 0;
  height: 2px;
  background: linear-gradient(90deg, var(--accent-teal), var(--accent-amber));
  z-index: 200;
  transition: width 0.1s linear;
}

/* ===== ANIMATIONS ===== */
@keyframes fadeInUp {
  from {
    opacity: 0;
    transform: translateY(16px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

.section {
  animation: fadeInUp 0.5s ease-out both;
}

.section:nth-child(2) { animation-delay: 0.05s; }
.section:nth-child(3) { animation-delay: 0.1s; }
.section:nth-child(4) { animation-delay: 0.15s; }

/* ===== RESPONSIVE ===== */
@media (max-width: 768px) {
  html { font-size: 15px; }
  .content { padding: 2rem 1.2rem 4rem; }
  .hero h1 { font-size: 2.2rem; }
  .chapter-header h1 { font-size: 1.8rem; }
  .top-nav { padding: 0.6rem 1rem; gap: 0.8rem; flex-wrap: wrap; }
  .top-nav .nav-brand { width: 100%; margin-bottom: 0.2rem; }
  .chapter-nav { flex-direction: column; }
  .chapter-nav a { max-width: 100%; }
  .def-box, .concept-box, .why-box, .important-box, .example-box, .step-box {
    padding: 1rem 1.2rem;
  }
}

/* ===== FOOTER ===== */
.footer {
  text-align: center;
  padding: 3rem 0;
  color: var(--text-muted);
  font-size: 0.78rem;
  font-family: 'JetBrains Mono', monospace;
}