:root {
  /* Light Mode */
  --bg: #f8fafc;
  --bg-card: #ffffff;
  --text: #1e293b;
  --text-secondary: #475569;
  --border: #e2e8f0;
  --primary: #1e40af;
  --primary-hover: #1d4ed8;
  --success: #10b981;
  --warning: #f59e0b;
  --danger: #ef4444;
  --gray-light: #f1f5f9;
  --gray-dark: #647481;
  --shadow: rgba(0, 0, 0, 0.05);
}

[data-theme="dark"] {
  /* Dark Mode */
  --bg: #0f172a;
  --bg-card: #1e293b;
  --text: #e2e8f0;
  --text-secondary: #94a3b8;
  --border: #334155;
  --primary: #3b82f6;
  --primary-hover: #60a5fa;
  --success: #34d399;
  --warning: #f59e0b;
  --danger: #f87171;
  --gray-light: #334155;
  --gray-dark: #94a3b8;
  --shadow: rgba(0, 0, 0, 0.3);
}

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

html {
  scroll-behavior: smooth;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, "Noto Sans", sans-serif;
  line-height: 1.6;
  color: var(--text);
  background-color: var(--bg);
  transition: background-color 0.3s, color 0.3s;
}

.container {
  width: 100%;
  max-width: 900px;
  margin: 0 auto;
  padding: 0 1.5rem;
}

/* Header */
.site-header {
  background-color: var(--bg-card);
  box-shadow: 0 2px 8px var(--shadow);
  position: sticky;
  top: 0;
  z-index: 100;
}

.site-header .container {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 1rem 0;
}

.site-header h1 {
  font-size: 1.875rem;
  font-weight: 700;
  color: var(--primary);
}

.slogan {
  font-size: 0.95rem;
  color: var(--text-secondary);
  margin-top: 0.25rem;
}

.top-nav ul {
  display: flex;
  list-style: none;
  gap: 1.2rem;
}

.top-nav a {
  text-decoration: none;
  color: var(--text);
  font-weight: 500;
  padding: 0.5rem 0.75rem;
  border-radius: 4px;
  transition: all 0.2s;
}

.top-nav a:hover,
.top-nav a.active {
  background-color: var(--primary);
  color: white;
}

.theme-toggle button {
  background: none;
  border: 1px solid var(--border);
  padding: 0.4rem 0.8rem;
  border-radius: 6px;
  cursor: pointer;
  font-size: 0.875rem;
  color: var(--text);
  transition: all 0.2s;
}

.theme-toggle button:hover {
  background-color: var(--gray-light);
}

/* Main Content */
.main-content {
  padding: 2rem 0;
}

.hero {
  text-align: center;
  margin-bottom: 2.5rem;
}

.hero h2 {
  font-size: 2.25rem;
  margin-bottom: 1rem;
  color: var(--primary);
}

.hero p {
  font-size: 1.1rem;
  color: var(--text-secondary);
  max-width: 650px;
  margin: 0 auto 1.5rem;
}

.cta-grid {
  display: flex;
  flex-wrap: wrap;
  gap: 1rem;
  justify-content: center;
}

.btn {
  padding: 0.75rem 1.5rem;
  border-radius: 6px;
  font-weight: 600;
  text-decoration: none;
  display: inline-block;
  transition: all 0.3s;
}

.btn.primary {
  background-color: var(--primary);
  color: white;
}

.btn.primary:hover {
  background-color: var(--primary-hover);
}

.btn.secondary {
  background-color: transparent;
  border: 2px solid var(--primary);
  color: var(--primary);
}

.btn.secondary:hover {
  background-color: var(--primary);
  color: white;
}

/* Chapter Cards */
.featured-chapters h3,
.features h3 {
  font-size: 1.5rem;
  margin: 2rem 0 1.5rem;
  color: var(--primary);
}

.chapter-cards {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
  gap: 1.5rem;
}

.card {
  background: var(--bg-card);
  border-radius: 8px;
  overflow: hidden;
  box-shadow: 0 4px 6px var(--shadow);
  border: 1px solid var(--border);
  transition: transform 0.3s, box-shadow 0.3s;
}

.card:hover {
  transform: translateY(-4px);
  box-shadow: 0 6px 12px var(--shadow);
}

.card h4 {
  font-size: 1.25rem;
  padding: 1.25rem 1.5rem 0.75rem;
  color: var(--primary);
}

.card p {
  padding: 0 1.5rem 1rem;
  color: var(--text-secondary);
  font-size: 0.95rem;
}

.card-link {
  display: block;
  padding: 0.75rem 1.5rem;
  color: var(--primary);
  text-decoration: none;
  font-weight: 600;
  background-color: var(--gray-light);
  text-align: center;
  transition: all 0.2s;
}

.card-link:hover {
  background-color: var(--primary);
  color: white;
}

/* Features List */
.feature-list {
  list-style: none;
  padding-left: 0;
}

.feature-list li {
  margin-bottom: 0.75rem;
  font-size: 1rem;
}

.feature-list strong {
  color: var(--primary);
}

/* Footer */
.site-footer {
  background-color: var(--bg-card);
  margin-top: 3rem;
  padding: 2rem 0;
  border-top: 1px solid var(--border);
}

.site-footer p {
  font-size: 0.875rem;
  color: var(--text-secondary);
  text-align: center;
}

.site-footer p:first-child {
  margin-bottom: 0.5rem;
}

/* Responsive */
@media (max-width: 768px) {
  .site-header .container {
    flex-direction: column;
    gap: 1rem;
  }

  .top-nav ul {
    flex-wrap: wrap;
    justify-content: center;
  }

  .cta-grid {
    flex-direction: column;
    align-items: stretch;
  }

  .btn {
    width: 100%;
    text-align: center;
  }
}