:root {
  --bg: #0f1419;
  --panel: #1a2030;
  --panel-2: #232b3d;
  --text: #e6edf3;
  --muted: #8b949e;
  --accent: #ff6b6b;
  --accent-2: #4ecdc4;
  --accent-3: #ffd93d;
  --good: #51cf66;
  --bad: #ff8787;
  --code-bg: #0d1117;
  --border: #30363d;
}
* { box-sizing: border-box; }
body {
  margin: 0;
  font-family: -apple-system, "Hiragino Sans", "Noto Sans JP", sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.7;
}
header {
  padding: 32px 24px;
  border-bottom: 4px solid var(--accent);
}
header h1 { margin: 0 0 8px; font-size: 24px; color: #fff; }
.meta {
  display: flex; flex-wrap: wrap; gap: 12px; align-items: center;
  font-size: 13px; color: rgba(255,255,255,0.9);
}
.badge {
  background: rgba(0,0,0,0.35); padding: 4px 10px; border-radius: 12px;
  border: 1px solid rgba(255,255,255,0.2); color: #fff;
}
.difficulty { color: var(--accent-3); letter-spacing: 1px; }

.phase-coding   { background: linear-gradient(135deg, #1864ab 0%, #5c7cfa 100%); }
.phase-infra    { background: linear-gradient(135deg, #137a8a 0%, #4ecdc4 100%); }
.phase-data     { background: linear-gradient(135deg, #5c3a1e 0%, #a05a2c 100%); }
.phase-biz      { background: linear-gradient(135deg, #1b7a3a 0%, #51cf66 100%); }
.phase-finance  { background: linear-gradient(135deg, #b8860b 0%, #ffd93d 100%); }
.phase-weakness { background: linear-gradient(135deg, #d9480f 0%, #ff922b 100%); }

.layout {
  display: grid;
  grid-template-columns: 240px 1fr;
  gap: 24px;
  max-width: 1280px;
  margin: 0 auto;
  padding: 24px;
}
nav.toc {
  position: sticky; top: 24px;
  height: max-content;
  background: var(--panel);
  border-radius: 8px;
  padding: 16px;
  border: 1px solid var(--border);
  font-size: 14px;
}
nav.toc h3 { margin: 0 0 12px; font-size: 13px; color: var(--muted); text-transform: uppercase; letter-spacing: 1px; }
nav.toc a {
  display: block; padding: 6px 10px;
  color: var(--text); text-decoration: none; border-radius: 4px;
  border-left: 2px solid transparent;
  transition: background 0.15s, border-color 0.15s;
}
nav.toc a:hover, nav.toc a.active { background: var(--panel-2); border-left-color: var(--accent-2); }
main { min-width: 0; }
section {
  background: var(--panel);
  border-radius: 8px;
  padding: 24px;
  margin-bottom: 20px;
  border: 1px solid var(--border);
}
section h2 {
  margin: 0 0 16px;
  color: var(--accent-2);
  border-bottom: 2px solid var(--border);
  padding-bottom: 8px;
  font-size: 20px;
}
section h3 { color: var(--accent-3); font-size: 16px; margin-top: 20px; }
code {
  background: var(--code-bg);
  padding: 2px 6px;
  border-radius: 3px;
  font-family: "JetBrains Mono", Menlo, monospace;
  font-size: 13px;
  color: #e1bee7;
}
pre {
  background: var(--code-bg);
  padding: 16px;
  border-radius: 6px;
  overflow-x: auto;
  border-left: 3px solid var(--accent-2);
  position: relative;
  margin: 0;
}
pre code { background: none; padding: 0; color: var(--text); font-size: 13px; }
.copy-btn {
  position: absolute; top: 8px; right: 8px;
  background: var(--panel-2); color: var(--muted);
  border: 1px solid var(--border); border-radius: 4px;
  padding: 4px 10px; font-size: 12px; cursor: pointer;
}
.copy-btn:hover { color: var(--accent-2); border-color: var(--accent-2); }

/* Tables */
table {
  width: 100%; border-collapse: collapse; margin: 12px 0;
  background: var(--code-bg); border-radius: 6px; overflow: hidden;
}
th, td {
  padding: 10px 14px; text-align: left; font-size: 14px;
  border-bottom: 1px solid var(--border);
}
th { background: var(--panel-2); color: var(--accent-3); }
tr:last-child td { border-bottom: none; }

/* Hints */
.hint {
  background: var(--panel-2);
  border-radius: 6px;
  margin-bottom: 8px;
  border-left: 3px solid var(--accent-3);
}
.hint summary {
  padding: 12px 16px;
  cursor: pointer;
  font-weight: 600;
  color: var(--accent-3);
  user-select: none;
}
.hint[open] summary { border-bottom: 1px solid var(--border); }
.hint-body { padding: 12px 16px; }

/* Code tabs */
.tab-bar {
  display: flex;
  gap: 4px;
  margin-bottom: -1px;
  border-bottom: 1px solid var(--border);
}
.tab-btn {
  padding: 8px 16px;
  background: var(--panel-2);
  border: 1px solid var(--border);
  border-bottom: none;
  border-radius: 6px 6px 0 0;
  color: var(--muted);
  cursor: pointer;
  font-size: 13px;
  transition: color 0.15s, background 0.15s;
}
.tab-btn.active {
  background: var(--code-bg);
  color: var(--accent-2);
  border-color: var(--border);
}
.tab-btn:hover:not(.active) { color: var(--text); }
.tab-content { display: none; }
.tab-content.active { display: block; }

/* Before/After comparison */
.before-after {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 12px;
  margin: 12px 0;
}
.before-panel, .after-panel {
  border-radius: 6px;
  overflow: hidden;
  border: 2px solid var(--border);
}
.before-panel { border-color: var(--bad); }
.after-panel  { border-color: var(--good); }
.panel-label {
  padding: 8px 12px;
  font-size: 12px;
  font-weight: 700;
  letter-spacing: 0.5px;
  display: flex;
  align-items: center;
  gap: 6px;
}
.before-panel .panel-label { background: #2d1414; color: var(--bad); }
.after-panel  .panel-label { background: #142d1c; color: var(--good); }

/* Problem tags */
.tag {
  display: inline-block;
  padding: 2px 8px;
  border-radius: 10px;
  font-size: 11px;
  font-weight: 700;
  letter-spacing: 0.3px;
}
.tag-cqs     { background: #862e9c33; color: #da77f2; border: 1px solid #862e9c; }
.tag-flag    { background: #d9480f33; color: #ff922b; border: 1px solid #d9480f; }
.tag-null    { background: #c9261433; color: #ff6b6b; border: 1px solid #c92614; }
.tag-float   { background: #b8860b33; color: #ffd93d; border: 1px solid #b8860b; }
.tag-immut   { background: #1864ab33; color: #74c0fc; border: 1px solid #1864ab; }

/* Principle cards */
.principle-cards {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
  gap: 12px;
  margin: 16px 0;
}
.principle-card {
  background: var(--code-bg);
  border-radius: 8px;
  padding: 16px;
  border: 1px solid var(--border);
  border-top: 3px solid var(--accent-2);
  transition: border-top-color 0.2s, transform 0.2s;
}
.principle-card:hover { transform: translateY(-2px); border-top-color: var(--accent); }
.principle-card h4 { margin: 0 0 6px; font-size: 14px; color: var(--accent-2); }
.principle-card p  { margin: 0; font-size: 13px; color: var(--muted); line-height: 1.5; }
.principle-icon { font-size: 24px; margin-bottom: 8px; }

/* Viz / SVG */
.viz {
  background: var(--code-bg);
  border-radius: 6px;
  padding: 16px;
  margin: 16px 0;
  border: 1px solid var(--border);
  overflow-x: auto;
}

/* Step */
.step {
  background: var(--code-bg);
  border-left: 3px solid var(--accent);
  padding: 12px 16px;
  margin: 12px 0;
  border-radius: 4px;
}
.step-num {
  display: inline-block;
  background: var(--accent); color: white;
  width: 28px; height: 28px; line-height: 28px; border-radius: 50%;
  text-align: center; font-weight: bold; margin-right: 8px;
}

/* Pill */
.pill { display: inline-block; padding: 2px 10px; border-radius: 10px; font-size: 12px; font-weight: 600; }
.pill-red    { background: #ff6b6b; color: white; }
.pill-green  { background: #51cf66; color: #000; }
.pill-yellow { background: #ffd93d; color: #000; }
.pill-blue   { background: #4ecdc4; color: #000; }

/* Self eval */
.self-eval {
  display: flex; gap: 16px; flex-wrap: wrap;
}
.self-eval label {
  background: var(--panel-2);
  padding: 10px 16px; border-radius: 20px;
  cursor: pointer; display: inline-flex; gap: 8px; align-items: center;
}
.self-eval input { accent-color: var(--accent-2); }
textarea {
  width: 100%; min-height: 80px;
  background: var(--code-bg); color: var(--text);
  border: 1px solid var(--border); border-radius: 4px;
  padding: 10px; font-family: inherit; font-size: 14px;
  resize: vertical;
}
.controls {
  display: flex; gap: 8px; flex-wrap: wrap;
  margin: 12px 0;
}
button.action {
  background: var(--accent-2); color: #000; border: none;
  padding: 8px 16px; border-radius: 4px; cursor: pointer;
  font-weight: 600;
}
button.action:hover { background: #6ee7df; }
button.action.secondary { background: var(--panel-2); color: var(--text); border: 1px solid var(--border); }

/* Callout boxes */
.callout {
  border-radius: 6px;
  padding: 12px 16px;
  margin: 12px 0;
  border-left: 4px solid;
  font-size: 14px;
}
.callout-warn { background: #2d1f0a; border-color: var(--accent-3); color: #ffe8a0; }
.callout-info { background: #0a1e2d; border-color: var(--accent-2); color: #a0e8e0; }
.callout-good { background: #0a2d14; border-color: var(--good);    color: #a0e8b0; }

/* Summary box */
.summary-box {
  background: linear-gradient(135deg, #1a2030, #232b3d);
  border: 1px solid var(--accent-2);
  border-radius: 8px;
  padding: 16px 20px;
  font-size: 14px;
  line-height: 1.8;
}

ul, ol { padding-left: 20px; }
li { margin: 4px 0; }
a { color: var(--accent-2); }
hr { border: none; border-top: 1px solid var(--border); margin: 24px 0; }

@media (max-width: 900px) {
  .layout { grid-template-columns: 1fr; }
  nav.toc { position: static; }
  .before-after { grid-template-columns: 1fr; }
  .principle-cards { grid-template-columns: 1fr; }
}