:root {
    --bg: #F4F6F4;
    --surface: #FFFFFF;
    --surface-2: #EAEFEA;
    --ink: #1A1F1C;
    --ink-soft: #5B655E;
    --border: #DCE3DD;
    --accent: #2E6F5E;
    --accent-ink: #FFFFFF;
    --accent-soft: #DEEDE7;
    --good: #237A4B;
    --good-soft: #DEF3E6;
    --bad: #B23A3A;
    --bad-soft: #FBE4E1;
    --warn: #9C6B14;
    --warn-soft: #F5EBD6;
    --font-display: -apple-system, "Segoe UI", ui-sans-serif, system-ui, sans-serif;
    --font-body: -apple-system, "Segoe UI", ui-sans-serif, system-ui, sans-serif;
    --font-mono: ui-monospace, "SF Mono", "Roboto Mono", "Consolas", monospace;
    --radius: 14px;
    --shadow: 0 1px 2px rgba(20, 30, 25, 0.06), 0 6px 20px rgba(20, 30, 25, 0.06);
  }

  @media (prefers-color-scheme: dark) {
    :root {
      --bg: #121815;
      --surface: #1B2320;
      --surface-2: #212B26;
      --ink: #EDF1EE;
      --ink-soft: #9FAAA3;
      --border: #2E3833;
      --accent: #6FC3A8;
      --accent-ink: #0C1613;
      --accent-soft: #1E332B;
      --good: #63C48A;
      --good-soft: #1B3327;
      --bad: #E38686;
      --bad-soft: #3A2323;
      --warn: #D8B168;
      --warn-soft: #362D19;
      --shadow: 0 1px 2px rgba(0, 0, 0, 0.3), 0 6px 24px rgba(0, 0, 0, 0.35);
    }
  }
  :root[data-theme="dark"] {
    --bg: #121815; --surface: #1B2320; --surface-2: #212B26; --ink: #EDF1EE; --ink-soft: #9FAAA3;
    --border: #2E3833; --accent: #6FC3A8; --accent-ink: #0C1613; --accent-soft: #1E332B;
    --good: #63C48A; --good-soft: #1B3327; --bad: #E38686; --bad-soft: #3A2323;
    --warn: #D8B168; --warn-soft: #362D19;
    --shadow: 0 1px 2px rgba(0,0,0,.3), 0 6px 24px rgba(0,0,0,.35);
  }
  :root[data-theme="light"] {
    --bg: #F4F6F4; --surface: #FFFFFF; --surface-2: #EAEFEA; --ink: #1A1F1C; --ink-soft: #5B655E;
    --border: #DCE3DD; --accent: #2E6F5E; --accent-ink: #FFFFFF; --accent-soft: #DEEDE7;
    --good: #237A4B; --good-soft: #DEF3E6; --bad: #B23A3A; --bad-soft: #FBE4E1;
    --warn: #9C6B14; --warn-soft: #F5EBD6;
    --shadow: 0 1px 2px rgba(20,30,25,.06), 0 6px 20px rgba(20,30,25,.06);
  }

  * { box-sizing: border-box; }
  html { -webkit-text-size-adjust: 100%; }
  body {
    margin: 0;
    background: var(--bg);
    color: var(--ink);
    font-family: var(--font-body);
    min-height: 100vh;
    padding: max(1rem, env(safe-area-inset-top)) max(1rem, env(safe-area-inset-right)) max(1.5rem, env(safe-area-inset-bottom)) max(1rem, env(safe-area-inset-left));
    display: flex;
    justify-content: center;
  }
  #app { width: 100%; max-width: 480px; display: flex; flex-direction: column; gap: 1rem; }

  h1, h2, h3 { font-family: var(--font-display); text-wrap: balance; margin: 0; }
  p { margin: 0; line-height: 1.5; }

  header.top {
    display: flex; flex-direction: column; gap: .6rem; width: 100%;
  }
  .brand {
    display: flex; align-items: baseline; justify-content: space-between; gap: .5rem;
  }
  .brand h1 { font-size: 1.15rem; font-weight: 800; letter-spacing: -0.01em; }
  .brand .tag {
    font-family: var(--font-mono); font-size: .72rem; color: var(--ink-soft);
    letter-spacing: .02em;
  }
  .progress-track {
    height: 6px; border-radius: 99px; background: var(--surface-2); overflow: hidden;
  }
  .progress-fill {
    height: 100%; background: var(--accent); border-radius: 99px;
    transition: width .25s ease;
    width: 0%;
  }
  .meta-row {
    display: flex; justify-content: space-between; font-family: var(--font-mono);
    font-size: .78rem; color: var(--ink-soft); font-variant-numeric: tabular-nums;
  }

  .card {
    background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius);
    box-shadow: var(--shadow); padding: 1.1rem;
  }

  .screen { display: none; flex-direction: column; gap: 1rem; width: 100%; }
  .screen.active { display: flex; }

  /* start screen */
  .intro p { color: var(--ink-soft); font-size: .95rem; }
  .mode-list { display: flex; flex-direction: column; gap: .6rem; }
  .mode-btn {
    text-align: left; border: 1px solid var(--border); background: var(--surface);
    border-radius: var(--radius); padding: .85rem 1rem; cursor: pointer; color: var(--ink);
    display: flex; flex-direction: column; gap: .2rem; font-family: var(--font-body);
    transition: border-color .15s ease, transform .1s ease;
  }
  .mode-btn:active { transform: scale(.98); }
  .mode-btn:hover { border-color: var(--accent); }
  .mode-btn .title { font-weight: 700; font-size: .98rem; }
  .mode-btn .sub { font-size: .8rem; color: var(--ink-soft); }
  .mode-btn.primary { background: var(--accent); border-color: var(--accent); color: var(--accent-ink); }
  .mode-btn.primary .sub { color: var(--accent-ink); opacity: .85; }

  .chips { display: flex; flex-wrap: wrap; gap: .5rem; }
  .chip {
    border: 1px solid var(--border); background: var(--surface-2); color: var(--ink);
    border-radius: 99px; padding: .4rem .75rem; font-size: .78rem; cursor: pointer;
    font-family: var(--font-body);
  }
  .chip:hover { border-color: var(--accent); }

  .stats-box { display: flex; flex-direction: column; gap: .55rem; }
  .stats-row {
    display: grid; grid-template-columns: 1fr auto; align-items: center; gap: .6rem; font-size: .85rem;
  }
  .stats-row .bar-track {
    grid-column: 1 / -1; height: 5px; background: var(--surface-2); border-radius: 99px; overflow: hidden;
  }
  .stats-row .bar-fill { height: 100%; border-radius: 99px; }
  .stats-pct { font-family: var(--font-mono); font-variant-numeric: tabular-nums; color: var(--ink-soft); }
  .empty-note { color: var(--ink-soft); font-size: .85rem; }
  .reset-link {
    background: none; border: none; color: var(--ink-soft); font-size: .75rem; text-decoration: underline;
    cursor: pointer; padding: 0; align-self: flex-start; font-family: var(--font-body);
  }
  .section-label {
    font-size: .78rem; color: var(--ink-soft); font-weight: 700; letter-spacing: .04em; text-transform: uppercase;
  }
  .col-stack { display: flex; flex-direction: column; gap: 1rem; }

  /* quiz screen */
  .topic-pill {
    display: inline-flex; align-self: flex-start; font-size: .72rem; font-weight: 600;
    letter-spacing: .02em; padding: .25rem .6rem; border-radius: 99px;
    background: var(--accent-soft); color: var(--accent);
  }
  .question-text { font-size: 1.05rem; font-weight: 600; line-height: 1.4; }
  .hint { font-size: .78rem; color: var(--ink-soft); }

  .options { display: flex; flex-direction: column; gap: .55rem; }
  .opt {
    display: flex; align-items: center; gap: .65rem; text-align: left;
    border: 1px solid var(--border); background: var(--surface); border-radius: 12px;
    padding: .8rem .9rem; cursor: pointer; font-family: var(--font-body); font-size: .93rem;
    color: var(--ink); min-height: 2.75rem; transition: border-color .15s ease, background .15s ease;
  }
  .opt:hover { border-color: var(--accent); }
  .opt .box {
    flex: none; width: 1.15rem; height: 1.15rem; border-radius: 5px; border: 1.5px solid var(--border);
    display: flex; align-items: center; justify-content: center;
  }
  .opt.bool .box, .opt.single .box { border-radius: 99px; }
  .opt .box::after { content: ""; width: .55rem; height: .55rem; border-radius: inherit; background: var(--accent); opacity: 0; }
  .opt.checked .box::after { opacity: 1; }
  .opt.locked { cursor: default; }
  .opt.correct { border-color: var(--good); background: var(--good-soft); }
  .opt.correct .box { border-color: var(--good); }
  .opt.correct .box::after { opacity: 1; background: var(--good); }
  .opt.wrong { border-color: var(--bad); background: var(--bad-soft); }
  .opt.wrong .box { border-color: var(--bad); }
  .opt.wrong .box::after { opacity: 1; background: var(--bad); }
  .opt.missed { border-color: var(--warn); background: var(--warn-soft); }
  .opt.missed .box { border-color: var(--warn); }

  .check-btn, .next-btn, .primary-btn {
    border: none; border-radius: 12px; padding: .85rem 1rem; font-weight: 700; font-size: .95rem;
    cursor: pointer; font-family: var(--font-body); background: var(--accent); color: var(--accent-ink);
  }
  .check-btn:disabled { opacity: .4; cursor: not-allowed; }
  .ghost-btn {
    border: 1px solid var(--border); background: transparent; color: var(--ink);
    border-radius: 12px; padding: .85rem 1rem; font-weight: 600; font-size: .9rem; cursor: pointer;
    font-family: var(--font-body);
  }

  .feedback {
    display: none; border-radius: 12px; padding: .85rem .95rem; font-size: .88rem; line-height: 1.5;
    border: 1px solid var(--border);
  }
  .feedback.show { display: block; }
  .feedback.correct { background: var(--good-soft); border-color: var(--good); color: var(--ink); }
  .feedback.wrong { background: var(--bad-soft); border-color: var(--bad); color: var(--ink); }
  .feedback .verdict { font-weight: 800; display: block; margin-bottom: .3rem; }
  .feedback.correct .verdict { color: var(--good); }
  .feedback.wrong .verdict { color: var(--bad); }

  .row-end { display: flex; gap: .6rem; }
  .row-end > * { flex: 1; }

  /* result screen */
  .score-hero { text-align: center; display: flex; flex-direction: column; gap: .3rem; padding: .5rem 0 .2rem; }
  .score-num { font-family: var(--font-display); font-size: 2.6rem; font-weight: 800; letter-spacing: -0.02em; }
  .score-sub { color: var(--ink-soft); font-size: .9rem; }

  footer.credit { text-align: center; font-size: .72rem; color: var(--ink-soft); padding-top: .3rem; }

  /* desktop */
  @media (min-width: 860px) {
    body { padding-top: 2.5rem; padding-bottom: 2.5rem; align-items: flex-start; }
    #app { max-width: 760px; gap: 1.4rem; }
    .narrow { max-width: 560px; margin-left: auto; margin-right: auto; width: 100%; }
    .start-grid {
      display: grid; grid-template-columns: 1.15fr 1fr; gap: 1.4rem; align-items: start;
    }
    .brand h1 { font-size: 1.4rem; }
    .question-text { font-size: 1.2rem; }
    .card { padding: 1.4rem; }
    .mode-btn { padding: 1rem 1.15rem; }
  }

  @media (prefers-reduced-motion: reduce) {
    * { transition-duration: 0.001ms !important; }
  }