:root {
  --bg: #0f1115;
  --bg-elev: #171a21;
  --bg-elev2: #1f232c;
  --border: #2a2f3a;
  --text: #d7dce5;
  --text-dim: #8b93a3;
  --accent: #4f9dff;
  --accent-soft: rgba(79, 157, 255, 0.15);
  --green: #3fb950;
  --red: #f85149;
  --code-bg: #11141a;
  --sidebar-w: 290px;
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  background: var(--bg);
  color: var(--text);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", "PingFang SC",
    "Hiragino Sans GB", "Microsoft YaHei", sans-serif;
  font-size: 15px;
  line-height: 1.7;
}

a { color: var(--accent); text-decoration: none; }
a:hover { text-decoration: underline; }
:focus-visible { outline: 3px solid var(--accent); outline-offset: 4px; }
#sidebarToggle { display: none; }

/* ---------- top bar ---------- */
.topbar {
  position: sticky;
  top: 0;
  height: 56px;
  display: flex;
  align-items: center;
  gap: 12px;
  padding: 0 18px;
  background: var(--bg-elev);
  border-bottom: 1px solid var(--border);
  z-index: 50;
}
.brand { font-weight: 600; font-size: 16px; letter-spacing: .2px; }
.spacer { flex: 1; }
.icon-btn {
  background: transparent;
  border: 1px solid var(--border);
  color: var(--text);
  border-radius: 6px;
  width: 36px; height: 36px;
  cursor: pointer;
  font-size: 16px;
}
.icon-btn:hover { background: var(--bg-elev2); }

.lang-switch { display: flex; border: 1px solid var(--border); border-radius: 8px; overflow: hidden; }
.lang-switch button {
  background: transparent;
  border: none;
  color: var(--text-dim);
  padding: 6px 14px;
  cursor: pointer;
  font-size: 14px;
}
.lang-switch button.active { background: var(--accent-soft); color: var(--accent); font-weight: 600; }
.lang-switch a { color: var(--text-dim); padding: 6px 14px; font-size: 14px; }
.lang-switch a.active { background: var(--accent-soft); color: var(--accent); font-weight: 600; }
.lang-switch a:hover { text-decoration: none; background: var(--bg-elev2); }
.site-link { font-size: 13px; }
.breadcrumbs { font-size: 13px; color: var(--text-dim); margin-bottom: 22px; }
.chapter-pagination { display: flex; justify-content: space-between; gap: 20px; margin-top: 36px; padding-top: 20px; border-top: 1px solid var(--border); }
.related-solution { padding: 14px; background: var(--bg-elev); border-radius: 8px; }
.content { min-width: 0; width: 100%; }
.content img { height: auto; }
.content h1, .content h2, .content h3 { scroll-margin-top: 72px; }

@media (max-width: 560px) {
  .topbar { gap: 6px; padding: 0 10px; }
  .brand { font-size: 13px; }
  .lang-switch a { padding: 6px 9px; }
  .site-link { font-size: 12px; }
  .chapter-pagination { flex-direction: column; }
}

/* ---------- layout ---------- */
.layout { display: flex; min-height: calc(100vh - 56px); }

.sidebar {
  width: var(--sidebar-w);
  flex: 0 0 var(--sidebar-w);
  background: var(--bg-elev);
  border-right: 1px solid var(--border);
  padding: 16px 10px;
  overflow-y: auto;
  position: sticky;
  top: 56px;
  height: calc(100vh - 56px);
}
.nav-group { margin-bottom: 18px; }
.nav-group-title {
  font-size: 12px;
  text-transform: uppercase;
  letter-spacing: .6px;
  color: var(--text-dim);
  padding: 6px 12px;
  font-weight: 600;
}
.nav-item {
  display: block;
  padding: 7px 12px;
  border-radius: 7px;
  color: var(--text);
  font-size: 14px;
  margin: 2px 0;
}
.nav-item:hover { background: var(--bg-elev2); text-decoration: none; }
.nav-item.active { background: var(--accent-soft); color: var(--accent); font-weight: 600; }

.sidebar-backdrop { display: none; }

.content {
  flex: 1;
  padding: 32px 48px 80px;
  max-width: 960px;
  margin: 0 auto;
}
.loading { color: var(--text-dim); padding: 40px 0; }

/* ---------- markdown content ---------- */
.content h1 { font-size: 28px; margin: 0 0 18px; border-bottom: 1px solid var(--border); padding-bottom: 12px; }
.content h2 { font-size: 22px; margin: 34px 0 14px; }
.content h3 { font-size: 18px; margin: 26px 0 10px; }
.content h4 { font-size: 15px; margin: 18px 0 8px; color: var(--text-dim); }
.content p { margin: 12px 0; overflow-wrap: anywhere; }
.content ul, .content ol { padding-left: 24px; }
.content li { margin: 6px 0; overflow-wrap: anywhere; }
.content a { text-decoration: underline; text-underline-offset: 3px; }
.content blockquote {
  border-left: 3px solid var(--accent);
  background: var(--bg-elev);
  margin: 16px 0;
  padding: 10px 16px;
  color: var(--text-dim);
  border-radius: 0 8px 8px 0;
}
.content code {
  background: var(--code-bg);
  border: 1px solid var(--border);
  border-radius: 5px;
  padding: 1px 6px;
  font-family: "SFMono-Regular", Consolas, monospace;
  font-size: 13px;
  color: #e6c07b;
}
.content pre {
  background: var(--code-bg);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 16px;
  overflow-x: auto;
}
.content pre code { background: transparent; border: none; padding: 0; color: var(--text); }
.content table {
  display: block;
  max-width: 100%;
  overflow-x: auto;
  border-collapse: collapse;
  width: 100%;
  margin: 18px 0;
  font-size: 14px;
}
.content th, .content td {
  border: 1px solid var(--border);
  padding: 9px 12px;
  text-align: left;
}
.content th { background: var(--bg-elev2); font-weight: 600; }
.content tr:nth-child(even) td { background: rgba(255,255,255,0.02); }
.content img { max-width: 100%; border: 1px solid var(--border); border-radius: 10px; margin: 12px 0; }

.lang-fallback {
  background: rgba(248, 81, 73, 0.1);
  border: 1px solid var(--red);
  color: #ffb4ae;
  padding: 8px 14px;
  border-radius: 8px;
  margin-bottom: 18px;
  font-size: 13px;
}

/* flowchart.js diagrams */
.fc-diagram {
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 22px;
  margin: 20px 0;
  overflow-x: auto;
  text-align: left;
}
.fc-diagram svg {
  display: inline-block;
}
.content .fc-hint { color: var(--text-dim); font-size: 13px; }

/* ---------- responsive ---------- */
@media (max-width: 860px) {
  #sidebarToggle { display: block; }
  .sidebar {
    position: fixed;
    left: 0; top: 56px;
    transform: translateX(-100%);
    transition: transform .2s ease;
    z-index: 40;
    box-shadow: 4px 0 16px rgba(0,0,0,.4);
  }
  .sidebar.open { transform: translateX(0); }
  .sidebar-backdrop.show {
    display: block;
    position: fixed; inset: 56px 0 0 0;
    background: rgba(0,0,0,.5); z-index: 39;
  }
  .content { padding: 24px 18px 60px; }
}
@media (prefers-reduced-motion: reduce) {
  .sidebar { transition: none; }
}