:root {
  --bg: #f6f7f5; --surface: #ffffff; --ink: #17201b; --muted: #5d6a63; --line: #dde2dd;
  --accent: #0f7a5f; --accent-ink: #ffffff; --good: #0f7a5f; --bad: #b4412e; --note: #eef2ee;
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root {
    --bg: #101512; --surface: #182019; --ink: #e8ede9; --muted: #9aa8a0; --line: #2b352e;
    --accent: #4cc3a1; --accent-ink: #06201a; --good: #4cc3a1; --bad: #ec8e7c; --note: #202a23;
    color-scheme: dark;
  }
}
* { box-sizing: border-box; }
body { margin: 0; background: var(--bg); color: var(--ink); font: 17px/1.55 system-ui, -apple-system, "Segoe UI", Roboto, sans-serif; }
main { max-width: 760px; margin: 0 auto; padding: 16px; }
.card { background: var(--surface); border: 1px solid var(--line); border-radius: 10px; padding: 18px; margin-bottom: 14px; }
h1 { font-size: 22px; line-height: 1.3; margin: 4px 0 12px; }
.meta { color: var(--muted); font-size: 14px; margin-bottom: 10px; }
.rich { overflow-wrap: anywhere; }
.rich p { margin: 0 0 12px; }
.rich img { max-width: 100%; height: auto; background: #fff; border-radius: 4px; display: block; margin: 10px 0; }
.rich .katex-display { overflow-x: auto; overflow-y: hidden; padding: 4px 0; }
.opt { display: flex; gap: 12px; align-items: flex-start; width: 100%; padding: 12px; margin-bottom: 8px; background: var(--surface); border: 1px solid var(--line); border-radius: 8px; cursor: pointer; }
.opt:has(input:checked) { border-color: var(--accent); background: var(--note); }
.opt input { margin-top: 5px; width: 18px; height: 18px; accent-color: var(--accent); flex: none; }
.opt b { flex: none; }
.opt .rich p { margin: 0; }
input[type=text], input[type=email] { width: 100%; font: inherit; padding: 12px; color: var(--ink); background: var(--surface); border: 1px solid var(--line); border-radius: 8px; }
button { font: inherit; font-weight: 600; min-height: 46px; padding: 10px 18px; border-radius: 8px; border: 1px solid var(--line); background: var(--surface); color: var(--ink); cursor: pointer; }
button.pri { background: var(--accent); border-color: var(--accent); color: var(--accent-ink); }
button:disabled { opacity: .55; cursor: default; }
button:focus-visible, input:focus-visible, a:focus-visible { outline: 3px solid var(--accent); outline-offset: 2px; }
.row { display: flex; flex-wrap: wrap; gap: 10px; align-items: center; margin-top: 14px; }
.note { color: var(--muted); font-size: 14px; margin: 8px 0 0; }
.result { font-weight: 700; font-size: 19px; margin: 0 0 8px; }
.result.correct { color: var(--good); }
.result.wrong { color: var(--ink); }
.notice { background: var(--note); border-radius: 8px; padding: 12px; margin-top: 12px; }
.err { color: var(--bad); margin-top: 10px; }
a { color: var(--accent); }
select { width: 100%; font: inherit; padding: 12px; color: var(--ink); background: var(--surface); border: 1px solid var(--line); border-radius: 8px; }
.free { margin-top: 18px; padding-top: 14px; border-top: 1px solid var(--line); }
.credit { color: var(--good); font-weight: 600; margin: 14px 0 0; }
table { width: 100%; border-collapse: collapse; font-size: 15px; font-variant-numeric: tabular-nums; }
th, td { text-align: left; padding: 7px 8px; border-bottom: 1px solid var(--line); }
th:not(:first-child), td:not(:first-child) { text-align: right; }
th { color: var(--muted); font-weight: 600; font-size: 13px; }
h2 { font-size: 17px; margin: 0 0 8px; }
.scroll { overflow-x: auto; }
.thin { color: var(--muted); font-style: italic; font-size: 13px; }
.stat { font-size: 26px; font-weight: 700; line-height: 1.2; }
.stats { display: grid; grid-template-columns: repeat(auto-fit, minmax(150px, 1fr)); gap: 14px; }
.stats .note { margin: 2px 0 0; }
ul.plain { margin: 0 0 12px; padding-left: 20px; }