:root {
  --bg: #f6f5f1; --card: #ffffff; --ink: #1d232b; --muted: #6a7280; --line: #e3e1da;
  --brand: #1f3a5f; --accent: #1a4fd6; --good: #1d7a46; --good-bg: #e7f4ec; --bad: #b4332b;
  --bad-bg: #fbecea; --warn-bg: #fff6e0; --radius: 14px;
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root { --bg: #14171c; --card: #1d2229; --ink: #e8eaed; --muted: #9aa3ae; --line: #2e353f;
    --brand: #9cc0ee; --good-bg: #173327; --bad-bg: #3a1d1b; --warn-bg: #3a3118; color-scheme: dark; }
}
* { box-sizing: border-box; }
body { margin: 0; background: var(--bg); color: var(--ink);
  font: 17px/1.5 -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif; }
main { max-width: 980px; margin: 0 auto; padding: 0 16px 80px; }
a { color: var(--accent); text-decoration: none; }
.top { position: sticky; top: 0; z-index: 10; display: flex; gap: 14px; align-items: baseline;
  padding: calc(env(safe-area-inset-top, 0px) + 12px) 16px 12px; background: var(--bg);
  border-bottom: 1px solid var(--line); }
.brand { font-weight: 700; color: var(--brand); font-size: 18px; }
.brand span { font-weight: 500; color: var(--muted); }
.crumb { color: var(--muted); font-size: 15px; }
.hero h1 { font-size: 28px; margin: 24px 0 4px; }
.muted { color: var(--muted); }
.card { background: var(--card); border: 1px solid var(--line); border-radius: var(--radius);
  padding: 6px 18px; margin: 14px 0; }
.card h2 { font-size: 17px; margin: 12px 0 4px; }
.rows { list-style: none; margin: 0; padding: 0; }
.rows li { display: flex; justify-content: space-between; align-items: center; gap: 12px;
  padding: 12px 0; border-top: 1px solid var(--line); }
.rows li:first-child { border-top: 0; }
.rows a { flex: 1; color: var(--ink); }
.code { color: var(--muted); font-variant-numeric: tabular-nums; margin-right: 6px; }
.count { color: var(--muted); font-size: 15px; white-space: nowrap; }
.score { font-weight: 600; color: var(--brand); }
.score.full { color: var(--good); }
.tabs { display: flex; gap: 8px; margin: 18px 0 4px; }
.tabs button, button, .button { font: inherit; border: 1px solid var(--line); background: var(--card);
  color: var(--ink); border-radius: 999px; padding: 9px 18px; cursor: pointer; min-height: 44px; }
.tabs button[aria-selected="true"], [aria-pressed="true"] { background: var(--brand); color: var(--bg); border-color: var(--brand); }
button.primary, .button.primary { background: var(--accent); color: #fff; border-color: var(--accent); font-weight: 600; }
button:disabled { opacity: .5; }
.toolbar { position: sticky; top: calc(env(safe-area-inset-top, 0px) + 50px); z-index: 9;
  display: flex; flex-wrap: wrap; gap: 8px; align-items: center; padding: 10px 0; background: var(--bg); }
.toolbar #submit { margin-left: auto; }
.pencil-hint { color: var(--muted); font-size: 14px; }
.stage { position: relative; background: #fff; border: 1px solid var(--line); border-radius: 8px;
  overflow: hidden; user-select: none; -webkit-user-select: none; }
.stage img { display: block; width: 100%; height: auto; }
.stage canvas { position: absolute; inset: 0; width: 100%; height: 100%; }
#live { touch-action: none; }
.flow { margin-top: 20px; }
.busy { display: flex; gap: 14px; align-items: center; padding: 18px; background: var(--card);
  border: 1px solid var(--line); border-radius: var(--radius); }
.spinner { width: 26px; height: 26px; border: 3px solid var(--line); border-top-color: var(--accent);
  border-radius: 50%; animation: spin 0.9s linear infinite; }
@keyframes spin { to { transform: rotate(360deg); } }
.error { padding: 14px 18px; background: var(--bad-bg); color: var(--bad); border-radius: var(--radius); }
.note { padding: 10px 14px; background: var(--warn-bg); border-radius: 10px; }
.part { border: 1px solid var(--line); border-radius: var(--radius); background: var(--card);
  margin: 14px 0; padding: 12px 16px; }
.part legend { font-weight: 600; padding: 0 6px; }
.part label { display: block; margin: 8px 0; font-size: 14px; color: var(--muted); }
.part textarea, .part input { display: block; width: 100%; margin-top: 4px; font: 16px/1.4 ui-monospace, Menlo, monospace;
  padding: 10px; border: 1px solid var(--line); border-radius: 8px; background: var(--bg); color: var(--ink); }
.preview { padding: 10px 12px; background: var(--bg); border-radius: 8px; overflow-x: auto; }
.preview .ans { margin-top: 6px; font-weight: 600; }
.total { font-size: 40px; font-weight: 700; color: var(--brand); margin: 6px 0 10px; }
.result { background: var(--card); border: 1px solid var(--line); border-radius: var(--radius); padding: 6px 18px 12px; margin: 14px 0; }
.result h3 { display: flex; justify-content: space-between; }
.marks { width: 100%; border-collapse: collapse; }
.marks td { vertical-align: top; padding: 8px 6px; border-top: 1px solid var(--line); }
.marks .mk { white-space: nowrap; font-weight: 700; width: 5.5em; }
.marks tr.got .mk { color: var(--good); }
.marks tr.lost .mk { color: var(--bad); }
.crit { font-size: 15px; }
.ev { font-size: 14px; color: var(--muted); font-style: italic; margin-top: 2px; }
.why { font-size: 15px; margin-top: 2px; }
.first-error { background: var(--bad-bg); padding: 10px 14px; border-radius: 10px; }
.warn { background: var(--warn-bg); padding: 4px 14px; border-radius: 10px; }
.feedback { font-size: 17px; }
.report { margin-top: 26px; }
.report textarea { width: 100%; margin: 8px 0; padding: 10px; border-radius: 8px; border: 1px solid var(--line);
  background: var(--card); color: var(--ink); font: inherit; }
.katex-display { overflow-x: auto; overflow-y: hidden; }
.logout { margin-left: auto; }
.logout button { padding: 4px 12px; font-size: 14px; color: var(--muted); }
.login { max-width: 360px; margin: 48px auto; display: flex; flex-direction: column; gap: 14px; }
.login h1 { font-size: 26px; margin: 0; }
.login label { display: flex; flex-direction: column; gap: 4px; font-weight: 600; font-size: 15px; }
.login input { font: inherit; font-weight: 400; padding: 10px 12px; border: 1px solid var(--line);
  border-radius: 10px; background: var(--card); color: var(--ink); }
.search { width: 100%; box-sizing: border-box; font: inherit; font-size: 17px; padding: 12px 14px; margin: 14px 0 4px;
  border: 1px solid var(--line); border-radius: 12px; background: var(--card); color: inherit; }
.navlink { margin-left: auto; font-weight: 600; }
.navlink + .logout { margin-left: 0; }
/* Show me how drawer */
.help { position: fixed; z-index: 30; top: 0; right: 0; bottom: 0; width: min(480px, 100vw); overflow-y: auto;
  background: var(--card); border-left: 1px solid var(--line); box-shadow: -8px 0 30px rgba(0,0,0,.18);
  padding: calc(env(safe-area-inset-top, 0px) + 10px) 18px calc(env(safe-area-inset-bottom, 0px) + 24px); }
.help header { display: flex; justify-content: space-between; align-items: center; position: sticky; top: -10px;
  background: var(--card); padding: 6px 0; z-index: 1; }
.help header h2 { margin: 0; font-size: 20px; }
@media (min-width: 1000px) { body.help-open main { margin-right: 490px; margin-left: 10px; } }
.sol-part h3 { margin: 18px 0 6px; }
.step { display: flex; gap: 12px; padding: 12px 0; border-top: 1px solid var(--line); }
.step-n { flex: none; width: 28px; height: 28px; border-radius: 50%; background: var(--brand); color: var(--bg);
  display: grid; place-items: center; font-weight: 700; font-size: 14px; }
.step-body { flex: 1; min-width: 0; }
.working { font-size: 18px; overflow-x: auto; }
.working p, .explain p { margin: 0 0 4px; }
.explain { color: var(--muted); font-size: 16px; margin-top: 4px; }
.earns span { display: inline-block; margin: 6px 6px 0 0; padding: 1px 8px; border-radius: 999px;
  background: var(--good-bg); color: var(--good); font-size: 13px; font-weight: 700; }
.step.answer { display: block; font-size: 18px; background: var(--bg); padding: 10px 12px; border-radius: 10px; border: 0; }
.step-nav { display: flex; gap: 8px; margin: 14px 0; }
.sol-end { margin: 14px 0; }
#helpChat h3 { margin: 22px 0 8px; }
/* chat */
.chat .msgs { display: flex; flex-direction: column; gap: 10px; }
.msg { padding: 10px 14px; border-radius: 14px; max-width: 92%; overflow-x: auto; }
.msg p { margin: 0 0 8px; } .msg p:last-child { margin: 0; }
.msg.user { align-self: flex-end; background: var(--accent); color: #fff; white-space: pre-wrap; }
.msg.assistant { align-self: flex-start; background: var(--bg); border: 1px solid var(--line); }
.msg.thinking { color: var(--muted); animation: pulse 1.2s ease-in-out infinite; }
@keyframes pulse { 50% { opacity: .4; } }
.msg.error { background: var(--bad-bg); color: var(--bad); }
.starters { display: flex; flex-wrap: wrap; gap: 8px; margin: 10px 0; }
.starters button { font-size: 15px; min-height: 38px; padding: 6px 14px; }
.composer { display: flex; gap: 8px; margin-top: 12px; align-items: flex-end; }
.composer textarea { flex: 1; font: inherit; padding: 10px 12px; border-radius: 12px; border: 1px solid var(--line);
  background: var(--bg); color: var(--ink); resize: vertical; }
.bookitem { margin: 10px 0; border: 1px solid var(--line); border-radius: 10px; overflow: hidden; background: #fff; }
.bookitem .bookhead { padding: 6px 10px; font-weight: 600; font-size: 15px; background: var(--card); color: var(--ink); }
.bookitem img { display: block; width: 100%; }
.bookitem details summary { padding: 6px 10px; cursor: pointer; background: var(--card); color: var(--accent); font-size: 15px; }
.qlink { display: inline-block; margin: 6px 0; font-weight: 600; }
.tutor-page { display: grid; gap: 18px; }
.tutor-page .chat { background: var(--card); border: 1px solid var(--line); border-radius: var(--radius); padding: 14px; }