:root {
  --bg: #f4f5f7;
  --surface: #ffffff;
  --text: #1b1f24;
  --muted: #5d6670;
  --line: #d9dde3;
  --primary: #1f5fbf;
  --primary-text: #ffffff;
  --ok: #1e7d3a;
  --ok-bg: #e3f4e8;
  --ng: #b3261e;
  --ng-bg: #fbe5e3;
  --sel: #dfe9fb;
  --img-filter: none;
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root {
    --bg: #111418;
    --surface: #1b1f25;
    --text: #e8eaed;
    --muted: #9aa3ad;
    --line: #2f3640;
    --primary: #5b9bff;
    --primary-text: #0b1220;
    --ok: #6fd28d;
    --ok-bg: #153222;
    --ng: #ff8a80;
    --ng-bg: #3a1a18;
    --sel: #22344f;
    --img-filter: invert(0.88) hue-rotate(180deg);
    color-scheme: dark;
  }
}

* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font-family: system-ui, -apple-system, "Hiragino Sans", "Noto Sans JP", "Yu Gothic UI", sans-serif;
  font-size: 17px;
  line-height: 1.7;
}
button, select { font: inherit; color: inherit; }

.topbar {
  position: sticky; top: 0; z-index: 10;
  display: flex; align-items: center; justify-content: space-between; gap: 8px;
  padding: calc(env(safe-area-inset-top) + 8px) 16px 8px;
  background: var(--surface);
  border-bottom: 1px solid var(--line);
}
.topbar-home {
  border: 0; background: none; padding: 6px 0;
  font-weight: 700; font-size: 18px; color: var(--primary);
}
.topbar-info { font-variant-numeric: tabular-nums; color: var(--muted); font-size: 15px; text-align: right; }
.topbar-info .timer { font-weight: 700; color: var(--text); font-size: 18px; }
.topbar-info .timer.low { color: var(--ng); }

.app {
  max-width: 640px; margin: 0 auto;
  padding: 16px 16px calc(env(safe-area-inset-bottom) + 24px);
}
.loading { color: var(--muted); }

h1, h2 { line-height: 1.35; }
h2 { font-size: 18px; margin: 24px 0 8px; }
.card {
  background: var(--surface);
  border: 1px solid var(--line);
  border-radius: 14px;
  padding: 16px;
  margin-bottom: 14px;
}
.card h2 { margin-top: 0; }
.muted { color: var(--muted); font-size: 14px; }
.row { display: flex; gap: 10px; align-items: center; }
.stack > * + * { margin-top: 10px; }

select {
  width: 100%; min-height: 48px; padding: 8px 12px;
  border: 1px solid var(--line); border-radius: 10px; background: var(--bg);
}
.btn {
  display: block; width: 100%; min-height: 54px;
  border: 1px solid var(--line); border-radius: 12px;
  background: var(--surface); padding: 10px 16px;
  font-weight: 700; font-size: 17px; cursor: pointer;
}
.btn.primary { background: var(--primary); color: var(--primary-text); border-color: var(--primary); }
.btn:disabled { opacity: 0.45; }
.btn.small { min-height: 44px; font-size: 15px; font-weight: 600; }
.btn-row { display: flex; gap: 10px; }
.btn-row > .btn { flex: 1; }

/* Question view */
.qhead { display: flex; justify-content: space-between; align-items: baseline; gap: 8px; margin-bottom: 8px; }
.qhead .qno { font-weight: 700; font-size: 18px; }
.source { font-size: 12.5px; color: var(--muted); }
.stem { white-space: pre-wrap; margin: 0 0 14px; }
.choices-text { list-style: none; margin: 0 0 12px; padding: 0; }
.choices-text li { display: flex; gap: 10px; padding: 8px 0; border-top: 1px dashed var(--line); }
.choices-text li:first-child { border-top: 0; }
.choices-text .lbl { flex: none; font-weight: 700; width: 1.4em; }
.qimg-wrap { margin: 0 0 12px; overflow-x: auto; -webkit-overflow-scrolling: touch; }
.qimg { display: block; width: 100%; height: auto; background: #fff; border-radius: 6px; filter: var(--img-filter); }
.qimg-wrap.zoom .qimg { width: 170%; max-width: none; }
.linkish { border: 0; background: none; color: var(--primary); padding: 6px 0; font-size: 14px; cursor: pointer; }

/* Answer pad: thumb-reachable, sticky at bottom */
.pad {
  position: sticky; bottom: 0; z-index: 5;
  margin: 0 -16px; padding: 10px 16px calc(env(safe-area-inset-bottom) + 10px);
  background: color-mix(in srgb, var(--bg) 92%, transparent);
  backdrop-filter: blur(6px);
  border-top: 1px solid var(--line);
}
.answers { display: grid; grid-template-columns: 1fr 1fr; gap: 10px; }
.answers .ans {
  min-height: 64px; border-radius: 14px; border: 2px solid var(--line);
  background: var(--surface); font-size: 26px; font-weight: 700; cursor: pointer;
}
.answers .ans.selected { background: var(--sel); border-color: var(--primary); }
.answers .ans.correct { background: var(--ok-bg); border-color: var(--ok); color: var(--ok); }
.answers .ans.wrong { background: var(--ng-bg); border-color: var(--ng); color: var(--ng); }
.pad .nav { display: flex; gap: 10px; margin-top: 10px; }
.pad .nav .btn { min-height: 48px; }

.feedback { border-radius: 12px; padding: 12px 14px; margin: 0 0 12px; font-weight: 700; }
.feedback.ok { background: var(--ok-bg); color: var(--ok); }
.feedback.ng { background: var(--ng-bg); color: var(--ng); }

/* Exam overview grid */
.grid { display: grid; grid-template-columns: repeat(5, 1fr); gap: 8px; }
.grid button {
  min-height: 48px; border-radius: 10px; border: 1px solid var(--line);
  background: var(--surface); font-weight: 700; cursor: pointer;
}
.grid button.answered { background: var(--sel); border-color: var(--primary); }
.grid button.current { outline: 3px solid var(--primary); outline-offset: 1px; }
.grid button.ok { background: var(--ok-bg); border-color: var(--ok); color: var(--ok); }
.grid button.ng { background: var(--ng-bg); border-color: var(--ng); color: var(--ng); }

/* Results */
.score { font-size: 44px; font-weight: 800; line-height: 1.1; font-variant-numeric: tabular-nums; }
.verdict { font-weight: 800; font-size: 20px; }
.verdict.pass { color: var(--ok); }
.verdict.fail { color: var(--ng); }
.bar { position: relative; height: 14px; border-radius: 7px; background: var(--line); margin: 14px 0 22px; }
.bar .fill { position: absolute; inset: 0 auto 0 0; border-radius: 7px; background: var(--primary); }
.bar .line { position: absolute; top: -6px; bottom: -6px; width: 2px; background: var(--text); }
.bar .line span { position: absolute; top: 20px; left: -14px; font-size: 12px; color: var(--muted); white-space: nowrap; }

table.stats { width: 100%; border-collapse: collapse; font-variant-numeric: tabular-nums; font-size: 15px; }
table.stats th, table.stats td { padding: 8px 4px; border-bottom: 1px solid var(--line); text-align: right; }
table.stats th:first-child, table.stats td:first-child { text-align: left; }

.notice { background: var(--sel); border-radius: 10px; padding: 10px 12px; font-size: 14px; }
.footer-note { margin-top: 28px; font-size: 12.5px; color: var(--muted); }

/* Explanation (AI-written, unverified) */
.expl { margin-top: 12px; scroll-margin-top: 72px; }
.expl-head { display: flex; align-items: center; gap: 8px; margin-bottom: 8px; }
.expl .badge { font-size: 12px; padding: 2px 8px; border-radius: 999px; border: 1px solid var(--ng); color: var(--ng); }
.expl p { line-height: 1.7; margin: 0 0 10px; }
.expl .tag { display: inline-block; font-size: 12px; font-weight: 700; margin-right: 6px; }
.expl .tag.ok { color: var(--ok); }
.expl .tag.ng { color: var(--ng); }