:root {
  --bg: #ffffff;
  --bg-soft: #f7f8fb;
  --bg-muted: #eef1f6;
  --fg: #1a2030;
  --fg-soft: #4a5568;
  --fg-muted: #6b7280;
  --border: #e5e7eb;
  --border-strong: #d1d5db;
  --accent: #2563eb;
  --accent-hover: #1d4ed8;
  --accent-soft: #eff6ff;
  --radius: 10px;
  --radius-sm: 6px;
  --shadow-sm: 0 1px 2px rgba(16, 24, 40, 0.05);
  --shadow: 0 4px 12px rgba(16, 24, 40, 0.08);
  --maxw: 1200px;
  --read: 72ch;
  --font: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  --mono: ui-monospace, SFMono-Regular, "SF Mono", Consolas, "Liberation Mono", monospace;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  font-family: var(--font);
  font-size: 16px;
  line-height: 1.65;
  color: var(--fg);
  background: var(--bg-soft);
  -webkit-font-smoothing: antialiased;
}
a { color: var(--accent); text-decoration: none; }
a:hover { color: var(--accent-hover); text-decoration: underline; }
img { max-width: 100%; height: auto; border-radius: var(--radius-sm); }

/* ==== Topbar ==== */
.topbar {
  background: #ffffff;
  border-bottom: 1px solid var(--border);
  position: sticky;
  top: 0;
  z-index: 50;
}
.topbar-inner {
  max-width: var(--maxw);
  margin: 0 auto;
  padding: 14px 24px;
  display: flex;
  align-items: center;
  gap: 24px;
}
.brand {
  display: flex;
  align-items: center;
  gap: 10px;
  font-weight: 700;
  font-size: 18px;
  color: var(--fg);
  letter-spacing: -0.01em;
}
.brand:hover { text-decoration: none; color: var(--accent); }
.brand-mark {
  width: 32px; height: 32px;
  border-radius: 8px;
  background: linear-gradient(135deg, #2563eb 0%, #7c3aed 100%);
  display: inline-flex; align-items: center; justify-content: center;
  color: #fff; font-weight: 700; font-size: 14px;
}
.primary-nav {
  display: flex;
  gap: 4px;
  margin-left: auto;
  flex-wrap: wrap;
}
.primary-nav > li { list-style: none; position: relative; }
.primary-nav a, .nav-item-btn {
  display: inline-block;
  padding: 8px 12px;
  font-size: 14.5px;
  font-weight: 500;
  color: var(--fg-soft);
  border-radius: var(--radius-sm);
  background: transparent;
  border: 0;
  cursor: pointer;
  white-space: nowrap;
}
.primary-nav a:hover, .nav-item-btn:hover {
  background: var(--bg-muted);
  color: var(--fg);
  text-decoration: none;
}
.primary-nav a.active { color: var(--accent); background: var(--accent-soft); }

/* Dropdowns */
.has-dropdown > .dropdown {
  display: none;
  position: absolute;
  top: calc(100% + 6px);
  left: 0;
  min-width: 260px;
  background: #fff;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 8px;
  z-index: 60;
}
.has-dropdown:hover > .dropdown,
.has-dropdown:focus-within > .dropdown { display: block; }
.dropdown li { list-style: none; }
.dropdown a {
  display: block;
  padding: 7px 10px;
  font-size: 14px;
  color: var(--fg-soft);
  border-radius: 6px;
}
.dropdown a:hover { background: var(--bg-muted); color: var(--accent); text-decoration: none; }
.dropdown-subtitle {
  padding: 8px 10px 4px;
  font-size: 11.5px;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--fg-muted);
}
.dropdown-divider {
  height: 1px;
  background: var(--border);
  margin: 6px 4px;
}

/* mobile nav toggle */
.nav-toggle { display: none; background: none; border: 1px solid var(--border); border-radius: 8px; padding: 8px 10px; cursor: pointer; margin-left: auto; }
@media (max-width: 960px) {
  .nav-toggle { display: block; }
  .primary-nav {
    display: none;
    position: absolute;
    top: 100%;
    left: 0; right: 0;
    flex-direction: column;
    gap: 0;
    background: #fff;
    border-bottom: 1px solid var(--border);
    padding: 12px 16px;
  }
  .primary-nav.open { display: flex; }
  .primary-nav > li { width: 100%; }
  .primary-nav a, .nav-item-btn { width: 100%; text-align: left; }
  .has-dropdown > .dropdown { position: static; display: block; box-shadow: none; border: 0; padding-left: 12px; }
}

/* ==== Page header ==== */
.page-hero {
  background: linear-gradient(180deg, #fff 0%, var(--bg-soft) 100%);
  border-bottom: 1px solid var(--border);
  padding: 28px 24px 24px;
}
.page-hero-inner {
  max-width: var(--maxw);
  margin: 0 auto;
}
.breadcrumbs {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
  font-size: 13.5px;
  color: var(--fg-muted);
  margin-bottom: 12px;
}
.breadcrumbs a { color: var(--fg-muted); }
.breadcrumbs a:hover { color: var(--accent); }
.breadcrumbs .sep { color: var(--border-strong); }
.page-title {
  font-size: clamp(24px, 3.6vw, 36px);
  line-height: 1.2;
  font-weight: 700;
  letter-spacing: -0.015em;
  margin: 0;
  color: var(--fg);
}
.page-subtitle {
  margin: 10px 0 0;
  color: var(--fg-muted);
  font-size: 15.5px;
}

/* ==== Layout ==== */
.main-wrap {
  max-width: var(--maxw);
  margin: 0 auto;
  padding: 28px 24px 64px;
  display: grid;
  grid-template-columns: minmax(0, 1fr) 280px;
  gap: 32px;
  align-items: start;
}
@media (max-width: 1024px) {
  .main-wrap { grid-template-columns: 1fr; }
  .toc { display: none; }
}

.article {
  background: #fff;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 32px 36px;
  box-shadow: var(--shadow-sm);
}
@media (max-width: 640px) {
  .article { padding: 20px; }
}

/* Article typography */
.article p { margin: 0 0 1.1em; }
.article h2 {
  font-size: 24px;
  font-weight: 700;
  letter-spacing: -0.01em;
  margin: 32px 0 14px;
  padding-bottom: 6px;
  border-bottom: 1px solid var(--border);
}
.article h3 {
  font-size: 19px;
  font-weight: 650;
  margin: 24px 0 10px;
}
.article h4 { font-size: 17px; font-weight: 600; margin: 20px 0 8px; }
.article ul, .article ol { padding-left: 1.4em; margin: 0 0 1.1em; }
.article li { margin-bottom: 0.3em; }
.article blockquote {
  margin: 1.2em 0;
  padding: 12px 18px;
  border-left: 3px solid var(--accent);
  background: var(--accent-soft);
  color: var(--fg);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  font-style: normal;
}
.article table {
  width: 100%;
  border-collapse: collapse;
  margin: 1.2em 0;
  font-size: 14.5px;
}
.article th, .article td {
  border: 1px solid var(--border);
  padding: 10px 12px;
  text-align: left;
  vertical-align: top;
}
.article th { background: var(--bg-muted); font-weight: 600; }
.article img { display: block; margin: 1em auto; max-width: 100%; border-radius: var(--radius-sm); }
.article code { font-family: var(--mono); font-size: 0.9em; background: var(--bg-muted); padding: 2px 6px; border-radius: 4px; }
.article pre { background: var(--bg-muted); padding: 14px 16px; border-radius: var(--radius-sm); overflow-x: auto; }
.article hr { border: 0; border-top: 1px solid var(--border); margin: 2em 0; }
.article iframe { max-width: 100%; border-radius: var(--radius-sm); }

/* Article meta & tags */
.article-meta {
  display: flex;
  gap: 14px;
  flex-wrap: wrap;
  color: var(--fg-muted);
  font-size: 13.5px;
  margin: -4px 0 20px;
  padding-bottom: 16px;
  border-bottom: 1px solid var(--border);
}
.article-category { color: var(--accent); font-weight: 500; }
.article-tags {
  margin-top: 28px;
  padding-top: 20px;
  border-top: 1px solid var(--border);
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
}
.article-tags a {
  display: inline-block;
  padding: 4px 10px;
  font-size: 13px;
  background: var(--bg-muted);
  color: var(--fg-soft);
  border-radius: 999px;
}
.article-tags a:hover { background: var(--accent-soft); color: var(--accent); text-decoration: none; }

/* ==== TOC Sidebar ==== */
.toc {
  position: sticky;
  top: 80px;
  background: #fff;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 18px 20px;
  box-shadow: var(--shadow-sm);
  max-height: calc(100vh - 100px);
  overflow-y: auto;
}
.toc-title {
  font-size: 12px;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--fg-muted);
  margin: 0 0 10px;
}
.toc ul { list-style: none; padding: 0; margin: 0; }
.toc li { margin: 4px 0; }
.toc a {
  display: block;
  padding: 4px 8px;
  font-size: 14px;
  color: var(--fg-soft);
  border-left: 2px solid transparent;
  border-radius: 0 4px 4px 0;
}
.toc a:hover { color: var(--accent); background: var(--accent-soft); border-left-color: var(--accent); text-decoration: none; }
.toc li.h3 a { padding-left: 20px; font-size: 13.5px; color: var(--fg-muted); }

/* ==== Category grid (for section index pages) ==== */
.card-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
  gap: 16px;
  margin: 24px 0;
}
.card {
  display: block;
  padding: 18px;
  background: #fff;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  color: var(--fg);
  box-shadow: var(--shadow-sm);
  transition: transform 0.15s ease, box-shadow 0.15s ease, border-color 0.15s ease;
}
.card:hover {
  transform: translateY(-2px);
  border-color: var(--accent);
  box-shadow: var(--shadow);
  text-decoration: none;
  color: var(--fg);
}
.card-title {
  font-size: 16px;
  font-weight: 600;
  margin: 0 0 6px;
  color: var(--fg);
}
.card-desc {
  font-size: 13.5px;
  color: var(--fg-muted);
  line-height: 1.5;
  margin: 0;
}

/* ==== Footer ==== */
.footer {
  background: #0f172a;
  color: #cbd5e1;
  padding: 40px 24px 24px;
  margin-top: 40px;
}
.footer-inner {
  max-width: var(--maxw);
  margin: 0 auto;
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
  gap: 32px;
}
.footer h4 {
  color: #fff;
  font-size: 14px;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  margin: 0 0 12px;
}
.footer ul { list-style: none; padding: 0; margin: 0; }
.footer li { margin-bottom: 7px; }
.footer a { color: #94a3b8; font-size: 14px; }
.footer a:hover { color: #fff; }
.footer-bottom {
  max-width: var(--maxw);
  margin: 28px auto 0;
  padding-top: 20px;
  border-top: 1px solid #1e293b;
  font-size: 13px;
  color: #64748b;
  text-align: center;
}

/* Skip link */
.skip-link { position: absolute; left: -9999px; }
.skip-link:focus { left: 16px; top: 16px; background: var(--accent); color: #fff; padding: 8px 12px; border-radius: 6px; z-index: 100; }

/* Utility */
.container { max-width: var(--maxw); margin: 0 auto; padding: 0 24px; }
.hidden { display: none !important; }