@import url('https://fonts.googleapis.com/css2?family=JetBrains+Mono:wght@400;600&family=Source+Serif+4:ital,wght@0,400;0,600;1,400&family=DM+Sans:wght@400;500;700&display=swap');

:root {
  --bg-primary: #0d1117;
  --bg-secondary: #161b22;
  --bg-tertiary: #21262d;
  --text-primary: #e6edf3;
  --text-secondary: #8b949e;
  --text-muted: #6e7681;
  --accent-purple: #a371f7;
  --accent-blue: #58a6ff;
  --accent-green: #3fb950;
  --accent-orange: #d29922;
  --accent-red: #f85149;
  --border-color: #30363d;
  --code-bg: #1a1f26;
  --highlight: rgba(163, 113, 247, 0.15);
}

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

html {
  scroll-behavior: smooth;
}

body {
  font-family: 'Source Serif 4', Georgia, serif;
  background: var(--bg-primary);
  color: var(--text-primary);
  line-height: 1.75;
  font-size: 17px;
}

/* Layout */
.container {
  max-width: 800px;
  margin: 0 auto;
  padding: 2rem;
}

/* Navigation */
.book-nav {
  position: fixed;
  top: 0;
  left: 0;
  width: 280px;
  height: 100vh;
  background: var(--bg-secondary);
  border-right: 1px solid var(--border-color);
  padding: 2rem 0;
  overflow-y: auto;
  z-index: 100;
}

.book-nav__logo {
  padding: 0 1.5rem 1.5rem;
  border-bottom: 1px solid var(--border-color);
  margin-bottom: 1rem;
}

.book-nav__logo h1 {
  font-family: 'DM Sans', sans-serif;
  font-size: 1.25rem;
  font-weight: 700;
  color: var(--accent-purple);
  letter-spacing: -0.02em;
}

.book-nav__logo span {
  font-size: 0.75rem;
  color: var(--text-muted);
  display: block;
  margin-top: 0.25rem;
}

.nav-section {
  padding: 0.75rem 1.5rem;
}

.nav-section__title {
  font-family: 'DM Sans', sans-serif;
  font-size: 0.7rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--text-muted);
  margin-bottom: 0.5rem;
}

.nav-section ul {
  list-style: none;
}

.nav-section li {
  margin: 0.2rem 0;
}

.nav-section a {
  display: block;
  padding: 0.4rem 0.75rem;
  color: var(--text-secondary);
  text-decoration: none;
  font-family: 'DM Sans', sans-serif;
  font-size: 0.875rem;
  border-radius: 6px;
  transition: all 0.15s ease;
}

.nav-section a:hover {
  color: var(--text-primary);
  background: var(--bg-tertiary);
}

.nav-section a.active {
  color: var(--accent-purple);
  background: var(--highlight);
}

/* Main content */
.main-content {
  margin-left: 280px;
  min-height: 100vh;
}

.chapter {
  padding: 3rem;
  max-width: 900px;
}

/* Typography */
h1, h2, h3, h4 {
  font-family: 'DM Sans', sans-serif;
  letter-spacing: -0.02em;
  color: var(--text-primary);
}

.chapter h1 {
  font-size: 2.5rem;
  font-weight: 700;
  margin-bottom: 0.5rem;
  background: linear-gradient(135deg, var(--accent-purple), var(--accent-blue));
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

.chapter__subtitle {
  font-size: 1.1rem;
  color: var(--text-secondary);
  margin-bottom: 2rem;
  padding-bottom: 2rem;
  border-bottom: 1px solid var(--border-color);
}

h2 {
  font-size: 1.5rem;
  margin: 2.5rem 0 1rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--border-color);
}

h2:first-of-type {
  border-top: none;
  padding-top: 0;
}

h3 {
  font-size: 1.2rem;
  margin: 2rem 0 0.75rem;
  color: var(--accent-blue);
}

h4 {
  font-size: 1rem;
  margin: 1.5rem 0 0.5rem;
  color: var(--text-secondary);
}

p {
  margin-bottom: 1.25rem;
}

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

em {
  font-style: italic;
}

/* Links */
a {
  color: var(--accent-blue);
  text-decoration: none;
  transition: color 0.15s ease;
}

a:hover {
  color: var(--accent-purple);
  text-decoration: underline;
}

/* Code */
code {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.875em;
  background: var(--code-bg);
  padding: 0.2em 0.4em;
  border-radius: 4px;
  color: var(--accent-orange);
}

pre {
  background: var(--code-bg);
  border: 1px solid var(--border-color);
  border-radius: 8px;
  padding: 1.25rem;
  margin: 1.5rem 0;
  overflow-x: auto;
}

pre code {
  background: none;
  padding: 0;
  color: var(--text-primary);
  font-size: 0.85rem;
  line-height: 1.6;
}

/* Syntax highlighting */
.keyword { color: var(--accent-purple); }
.function { color: var(--accent-blue); }
.string { color: var(--accent-green); }
.number { color: var(--accent-orange); }
.comment { color: var(--text-muted); font-style: italic; }
.atom { color: var(--accent-blue); }
.operator { color: var(--accent-red); }
.module { color: var(--accent-orange); }

/* Lists */
ul, ol {
  margin: 1rem 0 1.5rem 1.5rem;
}

li {
  margin: 0.5rem 0;
}

/* Callouts */
.callout {
  padding: 1.25rem 1.5rem;
  border-radius: 8px;
  margin: 1.5rem 0;
  border-left: 4px solid;
}

.callout--info {
  background: rgba(88, 166, 255, 0.1);
  border-color: var(--accent-blue);
}

.callout--warning {
  background: rgba(210, 153, 34, 0.1);
  border-color: var(--accent-orange);
}

.callout--tip {
  background: rgba(63, 185, 80, 0.1);
  border-color: var(--accent-green);
}

.callout--erlang {
  background: rgba(248, 81, 73, 0.1);
  border-color: var(--accent-red);
}

.callout__title {
  font-family: 'DM Sans', sans-serif;
  font-weight: 700;
  font-size: 0.85rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  margin-bottom: 0.5rem;
}

.callout--info .callout__title { color: var(--accent-blue); }
.callout--warning .callout__title { color: var(--accent-orange); }
.callout--tip .callout__title { color: var(--accent-green); }
.callout--erlang .callout__title { color: var(--accent-red); }

/* Exercise boxes */
.exercise {
  background: var(--bg-secondary);
  border: 1px solid var(--border-color);
  border-radius: 8px;
  padding: 1.5rem;
  margin: 2rem 0;
}

.exercise__header {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  margin-bottom: 1rem;
}

.exercise__number {
  background: var(--accent-purple);
  color: var(--bg-primary);
  font-family: 'DM Sans', sans-serif;
  font-weight: 700;
  font-size: 0.75rem;
  padding: 0.25rem 0.6rem;
  border-radius: 4px;
}

.exercise__title {
  font-family: 'DM Sans', sans-serif;
  font-weight: 600;
  color: var(--text-primary);
}

.exercise__difficulty {
  margin-left: auto;
  font-size: 0.75rem;
  color: var(--text-muted);
}

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

th, td {
  padding: 0.75rem 1rem;
  text-align: left;
  border-bottom: 1px solid var(--border-color);
}

th {
  font-family: 'DM Sans', sans-serif;
  font-weight: 600;
  color: var(--text-secondary);
  background: var(--bg-secondary);
}

/* Chapter navigation */
.chapter-nav {
  display: flex;
  justify-content: space-between;
  margin-top: 4rem;
  padding-top: 2rem;
  border-top: 1px solid var(--border-color);
}

.chapter-nav__link {
  display: flex;
  flex-direction: column;
  gap: 0.25rem;
  padding: 1rem;
  background: var(--bg-secondary);
  border: 1px solid var(--border-color);
  border-radius: 8px;
  text-decoration: none;
  transition: all 0.15s ease;
  max-width: 45%;
}

.chapter-nav__link:hover {
  border-color: var(--accent-purple);
  text-decoration: none;
}

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

.chapter-nav__label {
  font-family: 'DM Sans', sans-serif;
  font-size: 0.75rem;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

.chapter-nav__title {
  font-family: 'DM Sans', sans-serif;
  font-weight: 600;
  color: var(--accent-blue);
}

/* Index page specific */
.index-hero {
  padding: 4rem 3rem;
  text-align: center;
  border-bottom: 1px solid var(--border-color);
}

.index-hero h1 {
  font-size: 3rem;
  margin-bottom: 1rem;
}

.index-hero__tagline {
  font-size: 1.25rem;
  color: var(--text-secondary);
  max-width: 600px;
  margin: 0 auto 2rem;
}

.index-hero__meta {
  display: flex;
  justify-content: center;
  gap: 2rem;
  font-family: 'DM Sans', sans-serif;
  font-size: 0.875rem;
  color: var(--text-muted);
}

.phase-grid {
  display: grid;
  gap: 1.5rem;
  padding: 3rem;
}

.phase-card {
  background: var(--bg-secondary);
  border: 1px solid var(--border-color);
  border-radius: 12px;
  padding: 1.5rem;
  transition: all 0.2s ease;
}

.phase-card:hover {
  border-color: var(--accent-purple);
  transform: translateY(-2px);
}

.phase-card__number {
  font-family: 'DM Sans', sans-serif;
  font-size: 0.7rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--accent-purple);
  margin-bottom: 0.5rem;
}

.phase-card__title {
  font-family: 'DM Sans', sans-serif;
  font-size: 1.25rem;
  font-weight: 700;
  margin-bottom: 0.5rem;
}

.phase-card__desc {
  color: var(--text-secondary);
  font-size: 0.95rem;
  margin-bottom: 1rem;
}

.phase-card__chapters {
  list-style: none;
  margin: 0;
  padding: 0;
}

.phase-card__chapters li {
  margin: 0.3rem 0;
}

.phase-card__chapters a {
  font-family: 'DM Sans', sans-serif;
  font-size: 0.875rem;
  color: var(--text-secondary);
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

.phase-card__chapters a::before {
  content: '→';
  color: var(--accent-purple);
}

.phase-card__chapters a:hover {
  color: var(--accent-blue);
  text-decoration: none;
}

/* Time estimates */
.time-badge {
  display: inline-flex;
  align-items: center;
  gap: 0.3rem;
  font-family: 'DM Sans', sans-serif;
  font-size: 0.75rem;
  color: var(--text-muted);
  background: var(--bg-tertiary);
  padding: 0.2rem 0.5rem;
  border-radius: 4px;
}

/* Responsive */
@media (max-width: 900px) {
  .book-nav {
    transform: translateX(-100%);
    transition: transform 0.3s ease;
  }
  
  .book-nav.open {
    transform: translateX(0);
  }
  
  .main-content {
    margin-left: 0;
  }
  
  .chapter {
    padding: 2rem 1.5rem;
  }
}

/* Scrollbar */
::-webkit-scrollbar {
  width: 8px;
  height: 8px;
}

::-webkit-scrollbar-track {
  background: var(--bg-primary);
}

::-webkit-scrollbar-thumb {
  background: var(--border-color);
  border-radius: 4px;
}

::-webkit-scrollbar-thumb:hover {
  background: var(--text-muted);
}