:root {
  --bg: #f6f6fb; --surface: #ffffff; --surface-2: #eef0f7; --border: #dfe2ee;
  --text: #1b1d2a; --muted: #5b6074; --accent: #5b4dd6; --accent-soft: #ece9ff; --accent-ink: #ffffff;
  --ok: #12804c; --ok-bg: #e5f5ec; --bad: #c0362f; --bad-bg: #fcebea; --warn: #9a5b00; --warn-bg: #fff3dd;
  --cpp: #2463a6; --py: #2c7a4f; --xp: #a86d0b;
  --radius: 14px; --radius-sm: 9px;
  --shadow: 0 1px 2px rgba(20, 20, 50, .06), 0 6px 20px rgba(20, 20, 50, .06);
  --font: 'Inter', system-ui, -apple-system, 'Segoe UI', sans-serif;
  --mono: 'JetBrains Mono', ui-monospace, 'Cascadia Code', Consolas, monospace;
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme='light']) {
    --bg: #0f1117; --surface: #171a23; --surface-2: #1f2330; --border: #2b3040;
    --text: #e8eaf3; --muted: #9ba1b5; --accent: #8f85ff; --accent-soft: #262245; --accent-ink: #0f1117;
    --ok: #4fd18b; --ok-bg: #11301f; --bad: #ff7b72; --bad-bg: #3a1716; --warn: #f1b75e; --warn-bg: #33260f;
    --cpp: #79b6ff; --py: #6fd39c; --xp: #f6c453;
    --shadow: 0 1px 2px rgba(0, 0, 0, .4), 0 6px 20px rgba(0, 0, 0, .3);
    color-scheme: dark;
  }
}
:root[data-theme='dark'] {
  --bg: #0f1117; --surface: #171a23; --surface-2: #1f2330; --border: #2b3040;
  --text: #e8eaf3; --muted: #9ba1b5; --accent: #8f85ff; --accent-soft: #262245; --accent-ink: #0f1117;
  --ok: #4fd18b; --ok-bg: #11301f; --bad: #ff7b72; --bad-bg: #3a1716; --warn: #f1b75e; --warn-bg: #33260f;
  --cpp: #79b6ff; --py: #6fd39c; --xp: #f6c453;
  --shadow: 0 1px 2px rgba(0, 0, 0, .4), 0 6px 20px rgba(0, 0, 0, .3);
  color-scheme: dark;
}

* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body { margin: 0; background: var(--bg); color: var(--text); font: 16px/1.6 var(--font); min-height: 100vh; display: flex; flex-direction: column; }
a { color: var(--accent); }
[hidden] { display: none !important; }
code, pre, kbd { font-family: var(--mono); }
h1, h2, h3 { line-height: 1.25; }
.muted { color: var(--muted); }
.skip { position: absolute; left: -999px; }
.skip:focus { left: 16px; top: 8px; z-index: 10; background: var(--surface); padding: 8px 12px; border-radius: var(--radius-sm); }
:focus-visible { outline: 3px solid var(--accent); outline-offset: 2px; }

/* top bar */
.topbar { position: sticky; top: 0; z-index: 5; display: flex; align-items: center; gap: 20px; flex-wrap: wrap;
  padding: 10px 24px; background: color-mix(in srgb, var(--surface) 88%, transparent); backdrop-filter: blur(10px); border-bottom: 1px solid var(--border); }
.brand { display: flex; align-items: center; gap: 10px; font-weight: 700; color: var(--text); text-decoration: none; }
.brand-mark { display: grid; place-items: center; width: 32px; height: 32px; border-radius: 9px; background: var(--accent); color: var(--accent-ink); font-family: var(--mono); font-size: 14px; }
.nav { display: flex; gap: 4px; flex-wrap: wrap; }
.nav a { padding: 6px 12px; border-radius: 999px; color: var(--muted); text-decoration: none; font-weight: 500; }
.nav a:hover { color: var(--text); background: var(--surface-2); }
.nav a.active { color: var(--accent); background: var(--accent-soft); }
.topbar-right { margin-left: auto; display: flex; align-items: center; gap: 8px; flex-wrap: wrap; }
.pill { display: inline-block; min-width: 20px; padding: 0 6px; border-radius: 999px; background: var(--bad); color: #fff; font-size: 12px; text-align: center; }
.chip { padding: 4px 10px; border-radius: 999px; background: var(--surface-2); font-size: 14px; font-weight: 600; white-space: nowrap; }
.chip-xp { color: var(--xp); }
.seg { display: inline-flex; padding: 3px; border-radius: 999px; background: var(--surface-2); }
.seg button { border: 0; background: transparent; color: var(--muted); padding: 4px 10px; border-radius: 999px; font: 600 13px var(--font); cursor: pointer; }
.seg button[aria-checked='true'] { background: var(--surface); color: var(--text); box-shadow: var(--shadow); }
.icon-btn { border: 1px solid var(--border); background: var(--surface); color: var(--text); width: 34px; height: 34px; border-radius: 50%; cursor: pointer; font-size: 16px; }

main { flex: 1; width: 100%; max-width: 1180px; margin: 0 auto; padding: 28px 24px 64px; outline: none; }
.footer { text-align: center; color: var(--muted); font-size: 13px; padding: 24px 16px; border-top: 1px solid var(--border); }
.loading { text-align: center; color: var(--muted); padding: 48px 0; }

/* buttons */
.btn { display: inline-flex; align-items: center; gap: 8px; padding: 9px 16px; border-radius: 10px; border: 1px solid var(--border);
  background: var(--surface); color: var(--text); font: 600 15px var(--font); text-decoration: none; cursor: pointer; }
.btn:hover { border-color: var(--accent); }
.btn.primary { background: var(--accent); border-color: var(--accent); color: var(--accent-ink); }
.btn:disabled { opacity: .55; cursor: not-allowed; }
.btn.small { padding: 6px 12px; font-size: 14px; }
.btn.danger { color: var(--bad); }

/* dashboard */
.hero { display: grid; grid-template-columns: 1.4fr 1fr; gap: 20px 32px; align-items: center; padding: 28px; border-radius: var(--radius);
  background: linear-gradient(135deg, var(--accent-soft), var(--surface)); border: 1px solid var(--border); box-shadow: var(--shadow); }
.hero h1 { margin: 0 0 12px; font-size: 32px; }
.eyebrow { margin: 0; text-transform: uppercase; letter-spacing: .08em; font-size: 12px; font-weight: 700; color: var(--accent); }
.xpbar { height: 10px; border-radius: 999px; background: var(--surface-2); overflow: hidden; }
.xpbar span { display: block; height: 100%; background: linear-gradient(90deg, var(--accent), var(--xp)); border-radius: inherit; }
.hero-stats { display: flex; gap: 12px; flex-wrap: wrap; }
.stat { flex: 1; min-width: 90px; padding: 12px; border-radius: var(--radius-sm); background: var(--surface); border: 1px solid var(--border); display: flex; flex-direction: column; }
.stat-n { font-size: 22px; font-weight: 700; }
.stat-l { font-size: 13px; color: var(--muted); }
.hero-actions { grid-column: 1 / -1; display: flex; gap: 10px; flex-wrap: wrap; }
.tier { margin-top: 36px; }
.tier h2 { margin: 0; }
.tier > .muted { margin: 4px 0 14px; }
.grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(250px, 1fr)); gap: 14px; }
.card { display: flex; flex-direction: column; gap: 6px; padding: 16px; border-radius: var(--radius); background: var(--surface);
  border: 1px solid var(--border); box-shadow: var(--shadow); color: var(--text); text-decoration: none; transition: transform .12s, border-color .12s; }
a.card:hover { transform: translateY(-2px); border-color: var(--accent); }
.card h3 { margin: 4px 0 0; font-size: 17px; }
.card p { margin: 0; font-size: 14px; }
.card-top { display: flex; justify-content: space-between; align-items: center; }
.card-meta { color: var(--muted); font-size: 13px !important; margin-top: auto !important; padding-top: 6px; }
.card-mastered { border-color: var(--ok); }
.card-soon { opacity: .6; }
.num { font-family: var(--mono); font-weight: 600; color: var(--muted); }
.badges { display: flex; gap: 4px; }
.badge { font-size: 18px; }
.ring-track { fill: none; stroke: var(--surface-2); stroke-width: 4; }
.ring-fill { fill: none; stroke: var(--accent); stroke-width: 4; stroke-linecap: round; transition: stroke-dashoffset .4s; }
.card-mastered .ring-fill { stroke: var(--ok); }
.ring-text { font: 700 11px var(--font); fill: var(--text); }
.tag { display: inline-block; padding: 1px 8px; border-radius: 999px; font-size: 12px; font-weight: 700; background: var(--surface-2); }
.tag-cpp { color: var(--cpp); }
.tag-python { color: var(--py); }
.dots { color: var(--accent); letter-spacing: 2px; font-size: 11px; }
.empty { text-align: center; padding: 48px 16px; }

/* toasts */
#toasts { position: fixed; right: 16px; bottom: 16px; z-index: 20; display: flex; flex-direction: column; gap: 8px; align-items: flex-end; }
.toast { padding: 10px 14px; border-radius: 10px; background: var(--text); color: var(--bg); font-weight: 600; box-shadow: var(--shadow); transition: opacity .4s, transform .4s; }
.toast-xp { background: var(--xp); color: #1b1d2a; }
.toast-level, .toast-badge { background: var(--accent); color: var(--accent-ink); }
.toast.out { opacity: 0; transform: translateY(8px); }

@media (max-width: 760px) {
  .topbar { padding: 10px 16px; gap: 10px; }
  .topbar-right { margin-left: 0; width: 100%; }
  main { padding: 20px 16px 48px; }
  .hero { grid-template-columns: 1fr; padding: 20px; }
  .hero h1 { font-size: 26px; }
}

/* module page */
.module-head { display: flex; justify-content: space-between; gap: 20px; align-items: flex-start; flex-wrap: wrap; }
.module-head h1 { margin: 2px 0 6px; font-size: 30px; }
.module-head .muted { margin: 0; }
.module-mastery { display: flex; flex-direction: column; align-items: center; gap: 4px; font-size: 13px; text-align: center; }
.tabs { display: flex; gap: 4px; border-bottom: 1px solid var(--border); margin: 20px 0 24px; overflow-x: auto; }
.tab { padding: 10px 16px; color: var(--muted); text-decoration: none; font-weight: 600; border-bottom: 2px solid transparent; margin-bottom: -1px; white-space: nowrap; }
.tab:hover { color: var(--text); }
.tab.active { color: var(--accent); border-color: var(--accent); }
.module-pager { display: flex; justify-content: space-between; gap: 12px; margin-top: 40px; flex-wrap: wrap; }
.notice { padding: 12px 16px; border-radius: var(--radius-sm); }
.notice-bad { background: var(--bad-bg); color: var(--bad); }
.notice-warn { background: var(--warn-bg); color: var(--warn); }
.notice-ok { background: var(--ok-bg); color: var(--ok); }

/* lesson */
.lesson-layout { display: grid; grid-template-columns: 210px minmax(0, 1fr); gap: 36px; }
.toc { position: sticky; top: 84px; align-self: start; font-size: 14px; max-height: calc(100vh - 100px); overflow: auto; }
.toc button { display: block; width: 100%; padding: 4px 0; border: 0; background: none; color: var(--muted); font: inherit; text-align: left; cursor: pointer; }
.toc button:hover { color: var(--accent); }
.prose { max-width: 860px; }
.prose h1 { font-size: 28px; margin-top: 0; }
.prose h2 { margin-top: 2em; padding-top: .6em; border-top: 1px solid var(--border); }
.prose h3 { margin-top: 1.6em; }
.prose :not(pre) > code { background: var(--surface-2); padding: .1em .35em; border-radius: 5px; font-size: .88em; }
.prose pre, .quiz-code pre, .solution pre { background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius-sm); margin: 1em 0; overflow: auto; }
pre code.hljs, pre code { display: block; padding: 14px 16px; font-size: 14px; line-height: 1.55; background: transparent !important; }
.prose table { border-collapse: collapse; display: block; overflow-x: auto; }
.prose th, .prose td { border: 1px solid var(--border); padding: 6px 10px; text-align: left; vertical-align: top; }
.prose th { background: var(--surface-2); }
.prose blockquote { margin: 1.2em 0; padding: 10px 16px; border-left: 4px solid var(--border); background: var(--surface); border-radius: 0 var(--radius-sm) var(--radius-sm) 0; }
.prose blockquote p { margin: .4em 0; }
.prose .callout-note { border-left-color: var(--accent); background: var(--accent-soft); }
.prose .callout-pitfall { border-left-color: var(--bad); background: var(--bad-bg); }
.prose .callout-compare { border-left-color: var(--py); }
.prose .callout-deep { border-left-color: var(--xp); background: var(--warn-bg); }
.compare { display: grid; grid-template-columns: 1fr 1fr; gap: 12px; margin: 1em 0; }
.pane { min-width: 0; display: flex; flex-direction: column; }
.pane pre { margin: 0 !important; flex: 1; }
.pane-label { font: 700 12px var(--font); text-transform: uppercase; letter-spacing: .06em; padding: 0 2px 6px; }
.pane-cpp .pane-label { color: var(--cpp); }
.pane-python .pane-label { color: var(--py); }
body[data-lang='cpp'] .compare, body[data-lang='python'] .compare { grid-template-columns: 1fr; }
body[data-lang='cpp'] .pane-python, body[data-lang='python'] .pane-cpp { display: none; }
.lesson-next { display: flex; gap: 12px; align-items: center; flex-wrap: wrap; margin-top: 36px; padding: 20px; border-radius: var(--radius); background: var(--accent-soft); }
.lesson-next p { margin: 0 auto 0 0; font-weight: 600; }
.lesson { min-width: 0; }
@media (max-width: 960px) { .lesson-layout { grid-template-columns: minmax(0, 1fr); } .toc { position: static; max-height: none; } }
@media (max-width: 760px) { .compare { grid-template-columns: 1fr; } }

/* quiz */
.page-head h1 { margin: 0 0 6px; }
.quiz-intro, .quiz-card, .quiz-done { max-width: 860px; padding: 24px; border-radius: var(--radius); background: var(--surface); border: 1px solid var(--border); box-shadow: var(--shadow); }
.quiz-intro h2, .quiz-done h2 { margin-top: 0; }
.actions { display: flex; gap: 10px; flex-wrap: wrap; align-items: center; margin-top: 16px; }
.quiz-progress { display: flex; align-items: center; gap: 12px; font-size: 14px; color: var(--muted); }
.quiz-progress .bar { flex: 1; height: 6px; border-radius: 999px; background: var(--surface-2); overflow: hidden; }
.quiz-progress .bar span { display: block; height: 100%; background: var(--accent); }
.xp-session { color: var(--xp); font-weight: 700; }
.quiz-meta { display: flex; gap: 8px; align-items: center; margin: 14px 0 4px; flex-wrap: wrap; }
.quiz-prompt { font-size: 17px; }
.quiz-prompt p:first-child { margin-top: 8px; }
.options { display: flex; flex-direction: column; gap: 10px; margin-top: 12px; }
.option { display: flex; gap: 12px; align-items: flex-start; width: 100%; text-align: left; padding: 12px 14px; border-radius: 10px;
  border: 2px solid var(--border); background: var(--surface); color: var(--text); font: 15px/1.5 var(--font); cursor: pointer; }
.option:hover:not(:disabled) { border-color: var(--accent); }
.option:disabled { cursor: default; }
.option-key { flex: none; display: grid; place-items: center; width: 26px; height: 26px; border-radius: 7px; background: var(--surface-2); font-weight: 700; font-size: 13px; }
.option-text { min-width: 0; }
.option-text code { background: var(--surface-2); padding: .05em .3em; border-radius: 4px; }
.option-pre { margin: 0; padding: 0; white-space: pre-wrap; font-size: 14px; background: none; border: 0; }
.option.correct { border-color: var(--ok); background: var(--ok-bg); }
.option.wrong { border-color: var(--bad); background: var(--bad-bg); }
.option.other { opacity: .75; }
.why { margin: 6px 0 4px 40px; font-size: 14px; }
.why-ok strong { color: var(--ok); }
.why-bad strong { color: var(--bad); }
.verdict { margin-top: 18px; padding: 12px 14px; border-radius: 10px; font-weight: 700; }
.verdict .muted { font-weight: 400; }
.verdict-ok { background: var(--ok-bg); color: var(--ok); }
.verdict-bad { background: var(--bad-bg); color: var(--bad); }
.explanation { margin-top: 14px; padding: 4px 16px 8px; border-left: 4px solid var(--accent); background: var(--accent-soft); border-radius: 0 10px 10px 0; }
.explanation h3 { margin: 10px 0 0; font-size: 15px; }
.kbd-hint { font-size: 13px; }
.score { font-size: 28px; font-weight: 700; margin: 4px 0; }
.breakdown { border-collapse: collapse; width: 100%; margin: 12px 0; }
.breakdown th, .breakdown td { padding: 6px 10px; border-bottom: 1px solid var(--border); text-align: left; }
.missed ul { padding-left: 20px; }
@media (max-width: 760px) { .quiz-intro, .quiz-card, .quiz-done { padding: 16px; } .why { margin-left: 8px; } }

/* lab */
.lab-layout { display: grid; grid-template-columns: 250px minmax(0, 1fr); gap: 24px; }
.lab-list { min-width: 0; display: flex; flex-direction: column; gap: 6px; align-self: start; position: sticky; top: 84px; }
.lab-item { display: grid; grid-template-columns: 20px 1fr; gap: 2px 8px; padding: 10px 12px; border-radius: 10px; border: 1px solid var(--border);
  background: var(--surface); color: var(--text); text-decoration: none; font-size: 14px; }
.lab-item .tag, .lab-item .dots { grid-column: 2; justify-self: start; }
.lab-item.active { border-color: var(--accent); background: var(--accent-soft); }
.lab-status { color: var(--ok); font-weight: 700; grid-row: span 3; }
.lab-title { font-weight: 600; }
.lab-head { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; }
.lab-head h2 { margin: 0; }
.run-note { font-size: 13px; }
.editor { border: 1px solid var(--border); border-radius: var(--radius-sm); overflow: hidden; }
.editor .CodeMirror { height: auto; min-height: 260px; font: 14px/1.55 var(--mono); background: var(--surface); color: var(--text); }
.editor .CodeMirror-gutters { background: var(--surface-2); border-right: 1px solid var(--border); }
.editor .CodeMirror-linenumber { color: var(--muted); }
.editor .CodeMirror-cursor { border-left-color: var(--text); }
.editor .CodeMirror-selected, .editor .CodeMirror-focused .CodeMirror-selected { background: var(--accent-soft); }
:root[data-theme='dark'] .cm-keyword { color: #ff7b72; }
:root[data-theme='dark'] .cm-def, :root[data-theme='dark'] .cm-variable-2 { color: #d2a8ff; }
:root[data-theme='dark'] .cm-string { color: #a5d6ff; }
:root[data-theme='dark'] .cm-number, :root[data-theme='dark'] .cm-atom { color: #79c0ff; }
:root[data-theme='dark'] .cm-comment { color: #8b949e; }
:root[data-theme='dark'] .cm-type, :root[data-theme='dark'] .cm-builtin, :root[data-theme='dark'] .cm-meta { color: #ffa657; }
.editor-fallback { width: 100%; min-height: 320px; border: 0; padding: 12px; font: 14px/1.55 var(--mono); background: var(--surface); color: var(--text); resize: vertical; }
.toolbar { display: flex; gap: 8px; flex-wrap: wrap; align-items: center; margin: 12px 0; }
.run-status { font-size: 14px; }
.hint { margin: 8px 0; padding: 10px 14px; border-radius: 10px; background: var(--warn-bg); color: var(--text); }
.hint strong { color: var(--warn); }
.run-message { white-space: pre-wrap; padding: 12px; border-radius: 10px; background: var(--surface-2); font-size: 13px; overflow-x: auto; }
.test-list { list-style: none; padding: 0; margin: 10px 0; display: flex; flex-direction: column; gap: 6px; }
.test { display: flex; gap: 10px; flex-wrap: wrap; align-items: baseline; padding: 8px 12px; border-radius: 8px; background: var(--surface); border: 1px solid var(--border); }
.test-icon { font-weight: 700; }
.test-pass .test-icon { color: var(--ok); }
.test-fail .test-icon { color: var(--bad); }
.test-notrun { opacity: .7; }
.test-name { font-weight: 600; }
.test-msg { flex-basis: 100%; font: 13px/1.5 var(--mono); color: var(--muted); white-space: pre-wrap; }
.stdout pre { padding: 12px; border-radius: 10px; background: var(--surface-2); white-space: pre-wrap; }
.solution { margin-top: 24px; padding: 4px 20px 16px; border-radius: var(--radius); border: 1px solid var(--ok); background: var(--surface); }
.lab-main { min-width: 0; }
@media (max-width: 960px) { .lab-layout { grid-template-columns: minmax(0, 1fr); } .lab-list { position: static; flex-direction: row; overflow-x: auto; } .lab-item { min-width: 200px; } }

/* settings */
.settings-section { max-width: 760px; margin-bottom: 18px; padding: 20px 24px; border-radius: var(--radius); background: var(--surface); border: 1px solid var(--border); }
.settings-section h2 { margin: 0 0 4px; font-size: 18px; }
.radios { display: flex; gap: 8px 20px; flex-wrap: wrap; margin-top: 10px; }
.radio { display: flex; gap: 8px; align-items: center; cursor: pointer; }
.radio input { accent-color: var(--accent); width: 18px; height: 18px; }