:root {
  --bg: #f6f7fb;
  --panel: #ffffff;
  --text: #1c2030;
  --muted: #5d6479;
  --line: #e2e5ef;
  --accent: #f0b400;
  --accent-ink: #231a00;
  --brand: #4c5fd5;
  --code-bg: #14172a;
  --code-text: #e9ecff;
  --good: #1b9a55;
  --good-bg: #e3f6ec;
  --bad: #d23a4a;
  --bad-bg: #fde8ea;
  --tip-bg: #eaf0ff;
  --warn-bg: #fff4d6;
  --radius: 12px;
  --mono: ui-monospace, "SF Mono", Menlo, Consolas, "Liberation Mono", monospace;
}
@media (prefers-color-scheme: dark) {
  :root {
    --bg: #0f1220;
    --panel: #181c30;
    --text: #e8eaf6;
    --muted: #9aa1bd;
    --line: #2a3050;
    --brand: #8c9bff;
    --code-bg: #0a0c18;
    --good: #4cd68a;
    --good-bg: #123023;
    --bad: #ff7b88;
    --bad-bg: #3a1820;
    --tip-bg: #1a2447;
    --warn-bg: #3b3012;
  }
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font: 16px/1.65 system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
}
a { color: var(--brand); }
code { font-family: var(--mono); font-size: 0.9em; background: color-mix(in srgb, var(--brand) 14%, transparent); padding: 0.1em 0.4em; border-radius: 5px; }

/* ---------- top bar ---------- */
.topbar {
  position: sticky; top: 0; z-index: 20;
  display: flex; align-items: center; gap: 14px;
  padding: 10px 18px;
  background: var(--panel); border-bottom: 1px solid var(--line);
}
.brand { font-weight: 800; text-decoration: none; color: var(--text); display: flex; align-items: center; gap: 8px; font-size: 1.1rem; }
.logo { background: var(--accent); color: var(--accent-ink); border-radius: 6px; padding: 1px 7px; font-weight: 900; }
.icon-btn { display: none; background: none; border: 1px solid var(--line); color: var(--text); border-radius: 8px; font-size: 1.2rem; padding: 2px 10px; cursor: pointer; }
.overall { margin-left: auto; display: flex; align-items: center; gap: 10px; font-size: 0.85rem; color: var(--muted); }
.bar { width: 140px; height: 8px; background: var(--line); border-radius: 99px; overflow: hidden; }
.bar > div { height: 100%; width: 0; background: var(--good); transition: width 0.4s; }

/* ---------- layout ---------- */
.layout { display: grid; grid-template-columns: 290px 1fr; min-height: calc(100vh - 53px); }
.sidebar {
  position: sticky; top: 53px; align-self: start; height: calc(100vh - 53px); overflow-y: auto;
  padding: 16px 12px 40px; border-right: 1px solid var(--line); background: var(--panel);
}
.sidebar h4 { margin: 14px 10px 6px; font-size: 0.72rem; letter-spacing: 0.08em; text-transform: uppercase; color: var(--muted); }
.nav-item {
  display: flex; align-items: center; gap: 10px; padding: 8px 10px; border-radius: 9px;
  color: var(--text); text-decoration: none; font-size: 0.95rem;
}
.nav-item:hover { background: color-mix(in srgb, var(--brand) 10%, transparent); }
.nav-item.active { background: color-mix(in srgb, var(--brand) 18%, transparent); font-weight: 600; }
.nav-num {
  flex: none; width: 26px; height: 26px; border-radius: 50%; display: grid; place-items: center;
  font-size: 0.78rem; font-weight: 700; background: var(--line); color: var(--muted);
}
.nav-item.done .nav-num { background: var(--good); color: #fff; }
.nav-item small { margin-left: auto; color: var(--muted); font-size: 0.75rem; white-space: nowrap; }

.main { padding: 32px clamp(16px, 4vw, 56px) 90px; max-width: 920px; width: 100%; outline: none; }

/* ---------- content ---------- */
h1 { font-size: clamp(1.8rem, 4vw, 2.5rem); line-height: 1.2; margin: 0 0 6px; }
h2 { margin: 44px 0 10px; font-size: 1.5rem; }
h3 { margin: 30px 0 8px; font-size: 1.15rem; }
.meta { color: var(--muted); margin-bottom: 20px; }
.lead { font-size: 1.15rem; color: var(--muted); }
ul.list { padding-left: 1.2em; }
ul.list li { margin: 4px 0; }

.callout { padding: 12px 16px; border-radius: var(--radius); margin: 16px 0; border-left: 5px solid; }
.callout.tip { background: var(--tip-bg); border-color: var(--brand); }
.callout.warn { background: var(--warn-bg); border-color: var(--accent); }

pre.static, .editor {
  font-family: var(--mono); font-size: 0.88rem; line-height: 1.55;
  background: var(--code-bg); color: var(--code-text);
  border-radius: var(--radius); padding: 14px 16px; overflow-x: auto; margin: 0;
}

/* ---------- runnable example ---------- */
.example, .exercise { background: var(--panel); border: 1px solid var(--line); border-radius: var(--radius); margin: 16px 0; overflow: hidden; }
.ex-head { display: flex; align-items: center; gap: 8px; padding: 8px 12px; border-bottom: 1px solid var(--line); font-size: 0.82rem; color: var(--muted); }
.ex-head .spacer { flex: 1; }
.editor { width: 100%; display: block; border: 0; resize: vertical; white-space: pre; tab-size: 2; border-radius: 0; min-height: 70px; outline: none; }
.editor:focus { box-shadow: inset 0 0 0 2px var(--brand); }

.btn {
  font: inherit; font-size: 0.85rem; font-weight: 600; cursor: pointer;
  border: 1px solid var(--line); background: var(--panel); color: var(--text);
  border-radius: 8px; padding: 5px 12px;
}
.btn:hover { border-color: var(--brand); }
.btn.primary { background: var(--accent); border-color: var(--accent); color: var(--accent-ink); }
.btn.primary:hover { filter: brightness(1.05); }
.btn.ghost { color: var(--muted); }

.output { font-family: var(--mono); font-size: 0.85rem; padding: 10px 14px; background: color-mix(in srgb, var(--code-bg) 92%, #fff); color: #cfd4f5; white-space: pre-wrap; word-break: break-word; border-top: 1px solid var(--line); }
.output:empty { display: none; }
.output .err { color: #ff8b97; }
.output .dim { color: #7f87ad; }
.dom-playground { padding: 12px 16px; border-top: 1px dashed var(--line); background: var(--bg); min-height: 56px; }
.dom-playground:empty::before { content: "playground (empty)"; color: var(--muted); font-size: 0.8rem; }
.dom-playground .big { font-size: 1.4rem; font-weight: 700; }

/* ---------- exercises ---------- */
.exercise.passed { border-color: var(--good); }
.exercise .body { padding: 14px 16px 6px; }
.exercise .body p { margin: 0 0 10px; }
.ex-title { font-weight: 700; color: var(--text); }
.badge { font-size: 0.72rem; padding: 2px 8px; border-radius: 99px; background: var(--line); color: var(--muted); }
.exercise.passed .badge { background: var(--good-bg); color: var(--good); }
.actions small { color: var(--muted); }
.actions .spacer { flex: 1; }
.actions { display: flex; gap: 8px; flex-wrap: wrap; padding: 10px 12px; border-top: 1px solid var(--line); align-items: center; }
.results { list-style: none; margin: 0; padding: 0 16px; }
.results li { padding: 6px 0; border-top: 1px solid var(--line); font-size: 0.9rem; display: flex; gap: 10px; }
.results .mark { flex: none; font-weight: 800; }
.results .pass .mark { color: var(--good); }
.results .fail .mark { color: var(--bad); }
.results .msg { display: block; color: var(--bad); font-family: var(--mono); font-size: 0.82rem; margin-top: 2px; white-space: pre-wrap; }
.summary { margin: 0; padding: 10px 16px; font-weight: 600; }
.summary.good { background: var(--good-bg); color: var(--good); }
.summary.bad { background: var(--bad-bg); color: var(--bad); }
.hintbox { margin: 8px 12px 12px; padding: 10px 14px; border-radius: 10px; background: var(--warn-bg); font-size: 0.9rem; }
.hintbox pre { margin: 6px 0 0; white-space: pre-wrap; font-family: var(--mono); font-size: 0.83rem; }

/* ---------- quiz ---------- */
.quiz-q { background: var(--panel); border: 1px solid var(--line); border-radius: var(--radius); padding: 14px 16px; margin: 12px 0; }
.quiz-q p { margin: 0 0 10px; font-weight: 600; }
.opts { display: grid; gap: 8px; }
.opt { text-align: left; font: inherit; font-size: 0.93rem; padding: 8px 12px; border-radius: 9px; border: 1px solid var(--line); background: var(--bg); color: var(--text); cursor: pointer; font-family: var(--mono); }
.opt:hover:not(:disabled) { border-color: var(--brand); }
.opt.right { background: var(--good-bg); border-color: var(--good); }
.opt.wrong { background: var(--bad-bg); border-color: var(--bad); }
.why { margin: 10px 0 0; font-size: 0.9rem; color: var(--muted); }

/* ---------- home ---------- */
.hero { padding: 8px 0 8px; }
.cards { display: grid; grid-template-columns: repeat(auto-fill, minmax(230px, 1fr)); gap: 12px; margin-top: 14px; }
.card { background: var(--panel); border: 1px solid var(--line); border-radius: var(--radius); padding: 14px 16px; text-decoration: none; color: var(--text); display: block; }
.card:hover { border-color: var(--brand); transform: translateY(-1px); }
.card .n { color: var(--muted); font-size: 0.78rem; font-weight: 700; }
.card strong { display: block; margin: 2px 0 4px; }
.card small { color: var(--muted); }
.card.done { border-color: var(--good); }

.pager { display: flex; justify-content: space-between; gap: 12px; margin-top: 56px; flex-wrap: wrap; }
.pager a { text-decoration: none; }
.pager .btn { padding: 10px 18px; font-size: 0.95rem; }

.steps { display: grid; grid-template-columns: repeat(auto-fit, minmax(190px, 1fr)); gap: 12px; margin: 16px 0; }
.step { background: var(--panel); border: 1px solid var(--line); border-radius: var(--radius); padding: 12px 14px; font-size: 0.93rem; }
.step b { display: block; margin-bottom: 4px; }

/* ---------- mobile ---------- */
@media (max-width: 860px) {
  .layout { grid-template-columns: 1fr; }
  .icon-btn { display: block; }
  .sidebar { position: fixed; left: 0; top: 53px; width: min(310px, 86vw); z-index: 15; transform: translateX(-102%); transition: transform 0.2s; box-shadow: 6px 0 24px rgba(0,0,0,0.25); }
  body.menu-open .sidebar { transform: none; }
  .bar { width: 80px; }
}