:root {
  --bg: #f6f8fb;
  --surface: #ffffff;
  --surface-2: #eef2f8;
  --text: #1a2230;
  --muted: #5a6579;
  --border: #dbe1ea;
  --accent: #2f6fa7;
  --accent-ink: #ffffff;
  --accent-soft: #e4effa;
  --yellow: #ffd43b;
  --good: #1a7f4b;
  --good-soft: #e0f4e9;
  --bad: #b3261e;
  --bad-soft: #fbe6e4;
  --warn-soft: #fff4d1;
  --warn-edge: #d9a400;
  --hl: rgba(255, 212, 59, 0.38);
  --term-bg: #10151c;
  --term-text: #d8e0ec;
  --term-in: #7ee0a0;
  --term-err: #ff8f87;
  --term-dim: #7f8ba0;
  --code-bg: #fbfcfe;
  --shadow: 0 1px 2px rgba(20, 30, 50, .06), 0 4px 14px rgba(20, 30, 50, .05);
  --radius: 12px;
  --mono: ui-monospace, "SF Mono", "Cascadia Code", Menlo, Consolas, monospace;
  --sans: system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", sans-serif;
  /* editor syntax colours */
  --c-kw: #a626a4; --c-str: #2b7a3d; --c-num: #b45a00; --c-com: #7a8699;
  --c-def: #1b5fb4; --c-bi: #0b7285; --c-op: #4a5568;
}
:root[data-theme="dark"] {
  --bg: #0e131a; --surface: #161d28; --surface-2: #1d2634; --text: #e5eaf2; --muted: #9ba7ba;
  --border: #2a3547; --accent: #6fb0ea; --accent-ink: #0b1118; --accent-soft: #1b2a3c;
  --good: #5fd391; --good-soft: #13281d; --bad: #ff8f87; --bad-soft: #35191a;
  --warn-soft: #2f2711; --warn-edge: #e0b32a; --hl: rgba(255, 212, 59, 0.22);
  --code-bg: #121923; --shadow: 0 1px 2px rgba(0,0,0,.4), 0 4px 14px rgba(0,0,0,.25);
  --c-kw: #d58cf0; --c-str: #8ad3a0; --c-num: #f0b072; --c-com: #7f8ba0;
  --c-def: #7db8ff; --c-bi: #66d1e4; --c-op: #b6c0d0;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #0e131a; --surface: #161d28; --surface-2: #1d2634; --text: #e5eaf2; --muted: #9ba7ba;
    --border: #2a3547; --accent: #6fb0ea; --accent-ink: #0b1118; --accent-soft: #1b2a3c;
    --good: #5fd391; --good-soft: #13281d; --bad: #ff8f87; --bad-soft: #35191a;
    --warn-soft: #2f2711; --warn-edge: #e0b32a; --hl: rgba(255, 212, 59, 0.22);
    --code-bg: #121923; --shadow: 0 1px 2px rgba(0,0,0,.4), 0 4px 14px rgba(0,0,0,.25);
    --c-kw: #d58cf0; --c-str: #8ad3a0; --c-num: #f0b072; --c-com: #7f8ba0;
    --c-def: #7db8ff; --c-bi: #66d1e4; --c-op: #b6c0d0;
  }
}

* { box-sizing: border-box; }
html { scroll-padding-top: 72px; }
body {
  margin: 0; background: var(--bg); color: var(--text); font: 16px/1.65 var(--sans);
  -webkit-text-size-adjust: 100%;
}
a { color: var(--accent); }
code { font-family: var(--mono); font-size: .9em; background: var(--surface-2); padding: .1em .35em; border-radius: 5px; }
pre code { background: none; padding: 0; }
h1, h2, h3, h4 { line-height: 1.25; margin: 1.6em 0 .5em; }
h1 { font-size: 2rem; margin-top: .2em; }
h2 { font-size: 1.4rem; }
h3 { font-size: 1.12rem; }
p, ul, ol { margin: .7em 0; }
li { margin: .25em 0; }
.skip { position: absolute; left: -999px; top: 8px; background: var(--surface); padding: 8px 12px; z-index: 100; border-radius: 6px; }
.skip:focus { left: 8px; }
:focus-visible { outline: 3px solid var(--accent); outline-offset: 2px; border-radius: 4px; }

/* ---------- top bar ---------- */
.topbar {
  position: sticky; top: 0; z-index: 30; height: 56px; display: flex; align-items: center; gap: 14px;
  padding: 0 16px; background: var(--surface); border-bottom: 1px solid var(--border);
}
.brand { font-weight: 700; color: var(--text); text-decoration: none; font-size: 1.1rem; white-space: nowrap; }
.brand-mark {
  display: inline-block; background: #3776ab; color: var(--yellow); font-family: var(--mono);
  padding: 1px 7px; border-radius: 6px; margin-right: 4px; font-size: .95rem;
}
.topnav { display: flex; gap: 4px; margin-left: 8px; }
.topnav a { color: var(--muted); text-decoration: none; padding: 6px 10px; border-radius: 8px; font-weight: 500; }
.topnav a:hover, .topnav a.active { background: var(--surface-2); color: var(--text); }
.engine { margin-left: auto; display: flex; align-items: center; gap: 8px; font-size: .85rem; color: var(--muted); }
.engine .dot { width: 9px; height: 9px; border-radius: 50%; background: var(--warn-edge); }
.engine[data-state="ready"] .dot { background: var(--good); }
.engine[data-state="busy"] .dot { background: var(--accent); animation: pulse 1s infinite; }
.engine[data-state="loading"] .dot { animation: pulse 1s infinite; }
.engine[data-state="error"] .dot { background: var(--bad); }
@keyframes pulse { 50% { opacity: .35; } }
.icon-btn {
  border: 1px solid var(--border); background: var(--surface); color: var(--text); width: 38px; height: 38px;
  border-radius: 9px; font-size: 1.1rem; cursor: pointer;
}
.icon-btn:hover { background: var(--surface-2); }
.menu-btn { display: none; }

/* ---------- layout ---------- */
.layout { display: grid; grid-template-columns: 310px minmax(0, 1fr); min-height: calc(100vh - 56px); }
.sidebar {
  position: sticky; top: 56px; align-self: start; height: calc(100vh - 56px); overflow-y: auto;
  border-right: 1px solid var(--border); background: var(--surface); padding: 14px 10px 40px;
}
main { padding: 28px clamp(16px, 4vw, 48px) 80px; max-width: 980px; width: 100%; margin: 0 auto; outline: none; }
.scrim { display: none; }

.side-part { font-size: .72rem; letter-spacing: .08em; text-transform: uppercase; color: var(--muted); margin: 18px 10px 6px; font-weight: 700; }
.side-chapter > summary {
  list-style: none; cursor: pointer; padding: 7px 10px; border-radius: 8px; font-weight: 600; display: flex; gap: 8px; align-items: baseline;
}
.side-chapter > summary::-webkit-details-marker { display: none; }
.side-chapter > summary:hover { background: var(--surface-2); }
.side-chapter .num { color: var(--muted); font-variant-numeric: tabular-nums; min-width: 1.6em; font-weight: 500; }
.side-chapter .count { margin-left: auto; font-size: .75rem; color: var(--muted); font-weight: 500; }
.side-lessons { list-style: none; margin: 2px 0 8px; padding: 0 0 0 14px; border-left: 2px solid var(--border); margin-left: 18px; }
.side-lessons a {
  display: flex; gap: 8px; padding: 5px 10px; border-radius: 8px; text-decoration: none; color: var(--text); font-size: .92rem; line-height: 1.4;
}
.side-lessons a:hover { background: var(--surface-2); }
.side-lessons a.current { background: var(--accent-soft); color: var(--accent); font-weight: 600; }
.tick { width: 1.1em; flex: none; color: var(--muted); }
.done .tick, a.done .tick { color: var(--good); }

/* ---------- home ---------- */
.hero { background: linear-gradient(135deg, #2f6fa7 0%, #1d4e7e 100%); color: #fff; border-radius: 18px; padding: 34px clamp(20px, 4vw, 40px); box-shadow: var(--shadow); }
.hero h1 { margin: 0 0 8px; font-size: clamp(1.7rem, 4vw, 2.4rem); }
.hero p { max-width: 60ch; margin: 0 0 18px; color: #e3eefa; font-size: 1.05rem; }
.hero .btn { background: var(--yellow); color: #20283a; border-color: var(--yellow); }
.hero .stats { display: flex; gap: 22px; margin-top: 22px; flex-wrap: wrap; font-size: .9rem; color: #cfe2f6; }
.hero .stats b { display: block; font-size: 1.5rem; color: #fff; }
.how { display: grid; grid-template-columns: repeat(auto-fit, minmax(210px, 1fr)); gap: 14px; margin: 22px 0 6px; }
.how > div { background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius); padding: 14px 16px; }
.how h3 { margin: 0 0 4px; font-size: 1rem; }
.how p { margin: 0; color: var(--muted); font-size: .93rem; }
.chapter-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(260px, 1fr)); gap: 14px; }
.card {
  display: block; text-decoration: none; color: var(--text); background: var(--surface); border: 1px solid var(--border);
  border-radius: var(--radius); padding: 16px; box-shadow: var(--shadow);
}
.card:hover { border-color: var(--accent); }
.card .num { font-family: var(--mono); color: var(--accent); font-size: .85rem; font-weight: 700; }
.card h3 { margin: 2px 0 6px; font-size: 1.05rem; }
.card p { margin: 0 0 12px; color: var(--muted); font-size: .9rem; }
.bar { height: 7px; background: var(--surface-2); border-radius: 99px; overflow: hidden; }
.bar > i { display: block; height: 100%; background: var(--good); border-radius: 99px; }
.card small { color: var(--muted); display: block; margin-top: 6px; }

/* ---------- buttons ---------- */
.btn {
  display: inline-flex; align-items: center; gap: 6px; border: 1px solid var(--accent); background: var(--accent); color: var(--accent-ink);
  padding: 8px 16px; border-radius: 9px; font: 600 .93rem var(--sans); cursor: pointer; text-decoration: none;
}
.btn:hover { filter: brightness(1.08); }
.btn:disabled { opacity: .55; cursor: not-allowed; filter: none; }
.btn.ghost { background: transparent; color: var(--text); border-color: var(--border); }
.btn.ghost:hover { background: var(--surface-2); }
.btn.small { padding: 5px 11px; font-size: .85rem; }
.btn.danger { background: transparent; color: var(--bad); border-color: var(--bad); }

/* ---------- lesson page ---------- */
.crumbs { font-size: .88rem; color: var(--muted); }
.crumbs a { color: var(--muted); }
.lesson-meta { color: var(--muted); font-size: .92rem; display: flex; gap: 14px; flex-wrap: wrap; }
.goals { background: var(--accent-soft); border-radius: var(--radius); padding: 14px 18px; margin: 18px 0; }
.goals h2 { margin: 0 0 4px; font-size: .95rem; text-transform: uppercase; letter-spacing: .05em; color: var(--accent); }
.goals ul { margin: 4px 0 0; padding-left: 1.2em; }
.progress-line { display: flex; align-items: center; gap: 12px; font-size: .9rem; color: var(--muted); margin: 8px 0 0; }
.progress-line .bar { flex: 1; max-width: 260px; }
.prose { overflow-wrap: anywhere; }
.prose > :first-child { margin-top: 0; }
.prose table { border-collapse: collapse; margin: 1em 0; display: block; overflow-x: auto; }
.prose th, .prose td { border: 1px solid var(--border); padding: 6px 12px; text-align: left; }
.prose th { background: var(--surface-2); }
.prose blockquote { margin: 1em 0; padding: .1em 1em; border-left: 4px solid var(--border); color: var(--muted); }
pre.snippet {
  background: var(--code-bg); border: 1px solid var(--border); border-radius: 10px; padding: 12px 14px; overflow-x: auto;
  font: .9rem/1.55 var(--mono); margin: 1em 0;
}
.callout { border-left: 5px solid; border-radius: 8px; padding: 10px 16px; margin: 1.2em 0; }
.callout > :first-child { margin-top: 0; } .callout > :last-child { margin-bottom: 0; }
.callout.tip { background: var(--good-soft); border-color: var(--good); }
.callout.note { background: var(--accent-soft); border-color: var(--accent); }
.callout.warn { background: var(--warn-soft); border-color: var(--warn-edge); }
.callout.mistake { background: var(--bad-soft); border-color: var(--bad); }
.callout .label { font-weight: 700; font-size: .78rem; letter-spacing: .06em; text-transform: uppercase; display: block; margin-bottom: 2px; }
.lesson-nav { display: flex; justify-content: space-between; gap: 12px; margin-top: 40px; flex-wrap: wrap; }
.complete-row { margin-top: 36px; padding: 18px; border: 1px dashed var(--border); border-radius: var(--radius); display: flex; gap: 14px; align-items: center; flex-wrap: wrap; }

/* ---------- example widget ---------- */
.example, .exercise, .quiz {
  background: var(--surface); border: 1px solid var(--border); border-radius: 14px; box-shadow: var(--shadow);
  margin: 28px 0; overflow: hidden;
}
.block-head { display: flex; gap: 10px; align-items: center; padding: 12px 16px; border-bottom: 1px solid var(--border); background: var(--surface-2); }
.block-head h3 { margin: 0; font-size: 1.02rem; }
.tag { font-size: .7rem; font-weight: 800; letter-spacing: .07em; text-transform: uppercase; padding: 3px 8px; border-radius: 6px; background: var(--accent); color: var(--accent-ink); white-space: nowrap; }
.tag.ex { background: var(--good); color: var(--accent-ink); }
.tag.quiz { background: #8a5cd6; color: #fff; }
.tag.read { background: var(--muted); color: var(--surface); }
.block-body { padding: 14px 16px 18px; }
.block-body > .prose { margin-bottom: 12px; }
.tabs { display: flex; gap: 4px; flex-wrap: wrap; margin-bottom: 10px; }
.tabs button {
  border: 1px solid var(--border); background: var(--surface); color: var(--muted); padding: 4px 12px; border-radius: 99px; cursor: pointer; font: 600 .83rem var(--sans);
}
.tabs button[aria-selected="true"] { background: var(--accent); color: var(--accent-ink); border-color: var(--accent); }
.file-tabs { display: flex; gap: 0; border: 1px solid var(--border); border-bottom: 0; border-radius: 10px 10px 0 0; background: var(--surface-2); overflow-x: auto; }
.file-tabs button { border: 0; border-right: 1px solid var(--border); background: transparent; color: var(--muted); padding: 7px 14px; cursor: pointer; font: .85rem var(--mono); white-space: nowrap; }
.file-tabs button[aria-selected="true"] { background: var(--code-bg); color: var(--text); font-weight: 700; }
.ide { border: 1px solid var(--border); border-radius: 10px; overflow: hidden; }
.file-tabs + .ide { border-radius: 0 0 10px 10px; }
.ide .toolbar { display: flex; gap: 8px; align-items: center; padding: 8px 10px; background: var(--surface-2); border-top: 1px solid var(--border); flex-wrap: wrap; }
.ide .toolbar .spacer { flex: 1; }
.hint-key { font-size: .78rem; color: var(--muted); }
.stdin-box { padding: 8px 10px; border-top: 1px solid var(--border); background: var(--surface); }
.stdin-box label { font-size: .8rem; font-weight: 700; color: var(--muted); display: block; margin-bottom: 4px; }
.stdin-box textarea {
  width: 100%; min-height: 54px; resize: vertical; font: .9rem/1.5 var(--mono); background: var(--code-bg); color: var(--text);
  border: 1px solid var(--border); border-radius: 8px; padding: 6px 10px;
}
.stdin-box small { color: var(--muted); }
.output {
  background: var(--term-bg); color: var(--term-text); font: .88rem/1.55 var(--mono); padding: 10px 14px; min-height: 52px;
  max-height: 420px; overflow: auto; margin: 0; white-space: pre-wrap; word-break: break-word; border-top: 1px solid var(--border);
}
.output .in { color: var(--term-in); }
.output .err { color: var(--term-err); }
.output .dim { color: var(--term-dim); font-style: italic; }
.output img { display: block; max-width: 100%; background: #fff; border-radius: 6px; margin: 8px 0; }
.expected { margin-top: 12px; }
.expected summary, .more summary { cursor: pointer; font-weight: 600; color: var(--accent); }
.expected pre { background: var(--term-bg); color: var(--term-text); padding: 10px 14px; border-radius: 8px; font: .85rem/1.5 var(--mono); max-height: 260px; overflow: auto; white-space: pre-wrap; }
.explain { margin-top: 18px; }
.explain h4 { margin: 0 0 8px; font-size: .95rem; }
.explain ol { list-style: none; padding: 0; margin: 0; counter-reset: x; }
.explain li { margin: 0 0 6px; }
.explain button.step {
  display: grid; grid-template-columns: auto 1fr; gap: 12px; text-align: left; width: 100%; border: 1px solid var(--border); background: var(--surface);
  border-radius: 10px; padding: 9px 12px; cursor: pointer; font: inherit; color: inherit;
}
.explain button.step:hover { border-color: var(--accent); }
.explain button.step.active { border-color: var(--warn-edge); background: var(--warn-soft); }
.explain .lines { font: 700 .78rem var(--mono); background: var(--surface-2); color: var(--muted); border-radius: 6px; padding: 3px 8px; height: fit-content; white-space: nowrap; }
.explain .step.active .lines { background: var(--yellow); color: #20283a; }
.explain .body > :first-child { margin-top: 0; } .explain .body > :last-child { margin-bottom: 0; }
.explain .body p { margin: .35em 0; }
.explain .hint { font-size: .8rem; color: var(--muted); margin: 0 0 8px; }
.box-list { margin-top: 16px; border-radius: 10px; padding: 10px 16px; }
.box-list h4 { margin: 0 0 4px; font-size: .92rem; }
.box-list ul { margin: 4px 0; padding-left: 1.2em; }
.box-list.try { background: var(--accent-soft); }
.box-list.pitfall { background: var(--bad-soft); }
.readonly-note { font-size: .88rem; color: var(--muted); padding: 8px 10px; background: var(--surface-2); border-top: 1px solid var(--border); }
.readonly-note code { background: var(--surface); }

/* ---------- quiz & exercise ---------- */
.quiz .block-body label {
  display: flex; gap: 10px; align-items: flex-start; border: 1px solid var(--border); border-radius: 10px; padding: 9px 12px; margin: 6px 0; cursor: pointer;
}
.quiz label:hover { border-color: var(--accent); }
.quiz label.right { border-color: var(--good); background: var(--good-soft); }
.quiz label.wrong { border-color: var(--bad); background: var(--bad-soft); }
.quiz input { margin-top: 5px; }
.feedback { margin-top: 10px; padding: 10px 14px; border-radius: 10px; }
.feedback.good { background: var(--good-soft); border: 1px solid var(--good); }
.feedback.bad { background: var(--bad-soft); border: 1px solid var(--bad); }
.feedback > :first-child { margin-top: 0; } .feedback > :last-child { margin-bottom: 0; }
.exercise .tag { background: #d6602c; color: #fff; }
.exercise-actions { display: flex; gap: 8px; flex-wrap: wrap; margin-top: 12px; }
.result { margin-top: 12px; padding: 10px 14px; border-radius: 10px; font-weight: 600; }
.result.good { background: var(--good-soft); color: var(--good); border: 1px solid var(--good); }
.result.bad { background: var(--bad-soft); color: var(--bad); border: 1px solid var(--bad); }
.hints > div { margin-top: 8px; padding: 8px 12px; background: var(--warn-soft); border-left: 4px solid var(--warn-edge); border-radius: 6px; }
.hints > div > :first-child { margin-top: 0; } .hints > div > :last-child { margin-bottom: 0; }
.done-badge { margin-left: auto; font-size: .8rem; font-weight: 700; color: var(--good); }

/* ---------- CodeMirror (themed through variables) ---------- */
.CodeMirror { height: auto; font: .92rem/1.55 var(--mono); background: var(--code-bg); color: var(--text); }
.CodeMirror-scroll { max-height: 520px; min-height: 40px; }
.CodeMirror-gutters { background: var(--surface-2); border-right: 1px solid var(--border); }
.CodeMirror-linenumber { color: var(--muted); }
.CodeMirror-cursor { border-left: 2px solid var(--text); }
.CodeMirror-selected, .CodeMirror-focused .CodeMirror-selected { background: var(--accent-soft); }
.CodeMirror-matchingbracket { color: var(--text) !important; background: var(--hl); }
.CodeMirror .hl-line, .CodeMirror .hl-line .CodeMirror-gutter-elt { background: var(--hl); }
.cm-s-pa .cm-keyword { color: var(--c-kw); font-weight: 600; }
.cm-s-pa .cm-string, .cm-s-pa .cm-string-2 { color: var(--c-str); }
.cm-s-pa .cm-number { color: var(--c-num); }
.cm-s-pa .cm-comment { color: var(--c-com); font-style: italic; }
.cm-s-pa .cm-def { color: var(--c-def); }
.cm-s-pa .cm-builtin { color: var(--c-bi); }
.cm-s-pa .cm-operator, .cm-s-pa .cm-meta { color: var(--c-op); }
.cm-s-pa .cm-variable-2, .cm-s-pa .cm-variable-3 { color: var(--text); }
.cm-s-pa .cm-error { color: var(--bad); }
.snippet.cm-s-pa { color: var(--text); }

/* ---------- playground ---------- */
.playground-actions { display: flex; gap: 8px; margin: 10px 0; flex-wrap: wrap; }

/* ---------- small screens ---------- */
@media (max-width: 880px) {
  .layout { grid-template-columns: minmax(0, 1fr); }
  .menu-btn { display: inline-block; }
  .topnav { display: none; }
  .engine #engine-text { display: none; }
  .sidebar {
    position: fixed; left: 0; top: 56px; width: min(86vw, 330px); z-index: 40; transform: translateX(-102%);
    transition: transform .2s; box-shadow: var(--shadow);
  }
  body.menu-open .sidebar { transform: none; }
  body.menu-open .scrim { display: block; position: fixed; inset: 56px 0 0 0; background: rgba(0,0,0,.4); z-index: 35; }
  main { padding-top: 20px; }
}
@media (prefers-reduced-motion: reduce) { * { animation: none !important; transition: none !important; } }
.explain .body ul, .explain .body ol { padding-left: 1.2em; margin: .35em 0; }
.explain .body li { list-style: disc; }
.live-note { margin-top: 10px; padding: 8px 12px; border-radius: 8px; background: var(--warn-soft); border-left: 4px solid var(--warn-edge); font-size: .9rem; }
.card p { display: -webkit-box; -webkit-line-clamp: 4; -webkit-box-orient: vertical; overflow: hidden; }
.output:empty::before { content: "Press ▶ Run to see the output here."; color: var(--term-dim); font-style: italic; }