:root {
  --bg: #f8fafc;
  --bg-sidebar: #1e293b;
  --bg-sidebar-hover: #334155;
  --bg-card: #ffffff;
  --text: #1e293b;
  --text-light: #64748b;
  --text-sidebar: #cbd5e1;
  --text-sidebar-active: #f8fafc;
  --accent: #2563eb;
  --accent-light: #dbeafe;
  --accent-dark: #1d4ed8;
  --border: #e2e8f0;
  --green: #16a34a;
  --green-light: #dcfce7;
  --purple: #7c3aed;
  --purple-light: #ede9fe;
  --radius: 8px;
  --sidebar-w: 280px;
  --font-sans: "Segoe UI", "PingFang SC", "Microsoft YaHei", sans-serif;
  --font-mono: Consolas, "Fira Code", monospace;
}

* { box-sizing: border-box; margin: 0; padding: 0; }
html { scroll-behavior: smooth; }
body {
  font-family: var(--font-sans);
  color: var(--text);
  background: var(--bg);
  line-height: 1.75;
  display: flex;
  min-height: 100vh;
}

#progress-bar {
  position: fixed;
  top: 0;
  left: 0;
  width: 0;
  height: 3px;
  background: linear-gradient(90deg, var(--accent), var(--purple));
  z-index: 1000;
  transition: width .15s ease-out;
}

#sidebar {
  position: fixed;
  top: 0;
  left: 0;
  width: var(--sidebar-w);
  height: 100vh;
  background: var(--bg-sidebar);
  color: var(--text-sidebar);
  display: flex;
  flex-direction: column;
  overflow: hidden;
  z-index: 900;
  transition: transform .3s ease;
}

#sidebar .sidebar-header {
  padding: 20px 18px 12px;
  border-bottom: 1px solid rgba(255, 255, 255, .08);
}

#sidebar .sidebar-header a {
  display: inline-block;
  color: var(--text-sidebar);
  text-decoration: none;
  margin-bottom: 10px;
}

#sidebar .sidebar-header h2 {
  font-size: .9rem;
  color: var(--text-sidebar);
  margin-bottom: 10px;
}

#search-box {
  width: 100%;
  padding: 8px 12px;
  border: 1px solid rgba(255, 255, 255, .16);
  border-radius: 6px;
  background: rgba(255, 255, 255, .06);
  color: #fff;
}

#search-box::placeholder { color: rgba(255, 255, 255, .42); }

#sidebar nav { flex: 1; overflow-y: auto; padding: 10px 0; }
#sidebar nav ul { list-style: none; }

.nav-section-title {
  display: block;
  padding: 8px 18px 4px;
  font-size: .72rem;
  letter-spacing: .06em;
  color: rgba(255, 255, 255, .42);
}

#sidebar nav a {
  display: block;
  padding: 6px 18px 6px 24px;
  text-decoration: none;
  color: var(--text-sidebar);
  border-left: 3px solid transparent;
  transition: .15s;
  font-size: .88rem;
}

#sidebar nav a:hover {
  background: var(--bg-sidebar-hover);
  color: var(--text-sidebar-active);
}

#sidebar nav a.active {
  background: rgba(37, 99, 235, .15);
  color: var(--text-sidebar-active);
  border-left-color: var(--accent);
}

#sidebar-toggle {
  display: none;
  position: fixed;
  top: 10px;
  left: 10px;
  width: 40px;
  height: 40px;
  border: none;
  border-radius: 8px;
  background: var(--bg-sidebar);
  color: #fff;
  z-index: 950;
}

#main {
  margin-left: var(--sidebar-w);
  max-width: 900px;
  min-height: 100vh;
  padding: 40px 48px 80px;
}

h1 {
  font-size: 1.85rem;
  margin-bottom: 8px;
}

.page-subtitle {
  color: var(--text-light);
  margin-bottom: 28px;
  padding-bottom: 16px;
  border-bottom: 1px solid var(--border);
}

h2 {
  font-size: 1.35rem;
  margin: 36px 0 12px;
  padding-bottom: 6px;
  border-bottom: 2px solid var(--accent-light);
  scroll-margin-top: 80px;
}

h3 {
  font-size: 1.08rem;
  margin: 20px 0 8px;
  color: var(--accent-dark);
}

p, li { margin: 8px 0; }
ul, ol { margin: 8px 0 8px 24px; }

a { color: var(--accent); }

code {
  background: #eaf2ff;
  color: #1e3a8a;
  border: 1px solid #c7d2fe;
  border-radius: 6px;
  padding: 2px 6px;
  font-family: var(--font-mono);
  font-size: .9em;
}

pre {
  margin: 14px 0;
  border-radius: 8px;
  overflow-x: auto;
  background: #0f172a;
  color: #e2e8f0;
  border: 1px solid #1e293b;
}

pre code {
  display: block;
  padding: 14px;
  border: none;
  background: transparent;
  color: inherit;
}

.analogy-box {
  background: var(--green-light);
  border-left: 4px solid var(--green);
  border-radius: var(--radius);
  padding: 14px 18px;
  margin: 12px 0;
}

.key-insight {
  background: var(--purple-light);
  border-left: 4px solid var(--purple);
  border-radius: var(--radius);
  padding: 14px 18px;
  margin: 12px 0;
}

.search-highlight {
  background: #fef08a;
  border-radius: 2px;
  padding: 1px 2px;
}

.nav-hidden { display: none !important; }

@media (max-width: 860px) {
  #sidebar { transform: translateX(-100%); }
  #sidebar.open { transform: translateX(0); }
  #sidebar-toggle { display: block; }
  #main { margin-left: 0; padding: 24px 20px 60px; }
}