:root {
  /* 日间模式（默认）：白底黑字 + 蓝色边框（参考百词斩的清爽感） */
  --bg: #f8fafc;
  --bg-gradient: radial-gradient(1200px 800px at 10% 10%, #eff6ff, #f8fafc);
  --panel: #ffffff;
  --panel-soft: rgba(15, 23, 42, 0.03);
  --text: #0f172a;
  --muted: #475569;
  --primary: #2563eb;
  --primary-soft: rgba(37, 99, 235, 0.12);
  --success: #16a34a;
  --danger: #dc2626;
  --border: #3b82f6;
  --img-bg: #f1f5f9;
  --option-bg: linear-gradient(180deg, #ffffff, #f8fafc);
  --header-bg: linear-gradient(180deg, rgba(2, 6, 23, 0.03), rgba(2, 6, 23, 0.0));
  --controls-gradient: linear-gradient(180deg, rgba(248, 250, 252, 0), rgba(248, 250, 252, 0.96));
  --shadow-sm: 0 8px 24px rgba(2, 6, 23, 0.08);
}

html[data-theme="dark"] {
  /* 夜间模式：保留原来的配色 */
  --bg: #0b1020;
  --bg-gradient: radial-gradient(1200px 800px at 10% 10%, #0f1530, #0b1020);
  --panel: #121835;
  --panel-soft: rgba(255, 255, 255, 0.03);
  --text: #e7ecff;
  --muted: #9aa7d7;
  --primary: #6c8cff;
  --primary-soft: rgba(108, 140, 255, 0.14);
  --success: #22c55e;
  --danger: #ef4444;
  --border: #22306b;
  --img-bg: #0e1430;
  --option-bg: linear-gradient(180deg, #1a2350, #12183a);
  --header-bg: linear-gradient(180deg, rgba(255, 255, 255, 0.03), rgba(0, 0, 0, 0.0));
  --controls-gradient: linear-gradient(180deg, rgba(11, 16, 32, 0), rgba(11, 16, 32, 0.9));
  --shadow-sm: 0 8px 24px rgba(23, 40, 120, 0.35);
}

* { box-sizing: border-box; }

html, body { height: 100%; }
body {
  margin: 0;
  font-family: 'Inter', system-ui, -apple-system, Segoe UI, Roboto, Helvetica, Arial, 'Noto Sans', 'Apple Color Emoji', 'Segoe UI Emoji';
  color: var(--text);
  background: var(--bg-gradient);
}

#app { min-height: 100%; display: flex; flex-direction: column; }

.app-header, .app-footer {
  padding: 16px 20px;
  background: var(--header-bg);
}
.app-header { display: flex; align-items: center; justify-content: space-between; gap: 16px; }
.app-header h1 { margin: 0; font-weight: 800; font-size: 22px; letter-spacing: .3px; }
.mode { display: flex; align-items: center; gap: 8px; color: var(--muted); }
.mode select { background: var(--panel); color: var(--text); border: 1px solid var(--border); border-radius: 10px; padding: 8px 10px; }

.quiz {
  width: min(920px, 94%);
  margin: 0 auto;
  flex: 1;
  display: flex;
  flex-direction: column;
  gap: 16px;
  /* 预留出底部按钮区域的高度，避免内容被贴底按钮遮挡 */
  padding-bottom: 90px;
}

.stem { background: var(--panel); border: 1px solid var(--border); border-radius: 16px; padding: 16px; display: grid; gap: 10px; box-shadow: var(--shadow-sm); }
.stem-label { font-size: 13px; color: var(--muted); }
.stem-content { display: flex; align-items: center; justify-content: center; min-height: 180px; }
.stem-content img { max-width: 100%; max-height: 240px; object-fit: contain; border-radius: 10px; border: 1px solid var(--border); background: var(--img-bg); }

.options { display: grid; grid-template-columns: 1fr 1fr; gap: 12px; }
.option {
  background: var(--option-bg);
  color: var(--text);
  border: 1px solid var(--border);
  border-radius: 14px;
  padding: 14px 16px;
  font-size: 16px;
  text-align: left;
  cursor: pointer;
  transition: transform .06s ease, border-color .2s ease, box-shadow .2s ease;
}
.option:hover { transform: translateY(-1px); border-color: var(--primary); box-shadow: var(--shadow-sm); }
.option:active { transform: translateY(0); }

.feedback { background: var(--panel); border: 1px solid var(--border); border-radius: 16px; padding: 16px; display: grid; gap: 12px; box-shadow: var(--shadow-sm); }
.result { font-weight: 600; }
.result.ok { color: var(--success); }
.result.no { color: var(--danger); }
.answer-card { display: grid; grid-template-columns: 180px 1fr; gap: 16px; align-items: center; }
.answer-card img { width: 100%; border-radius: 12px; border: 1px solid var(--border); background: var(--img-bg); }
.answer-text { color: var(--muted); line-height: 1.6; }

.controls { display: flex; justify-content: space-between; margin-top: auto; position: sticky; bottom: 0; background: var(--controls-gradient); padding-top: 8px; }
.next { background: var(--primary); color: white; border: 1px solid transparent; border-radius: 12px; padding: 12px 16px; font-weight: 600; cursor: pointer; }
.next.secondary { background: var(--panel); color: var(--text); border-color: var(--border); }
.next.secondary:hover { border-color: var(--primary); box-shadow: var(--shadow-sm); }

/* 主页右上角主题切换按钮 */
.theme-toggle {
  position: fixed;
  top: 14px;
  right: 14px;
  z-index: 999;
  background: var(--panel);
  color: var(--text);
  border: 1px solid var(--border);
  border-radius: 999px;
  padding: 10px 12px;
  font-weight: 600;
  font-size: 13px;
  cursor: pointer;
  box-shadow: var(--shadow-sm);
}
.theme-toggle:hover { border-color: var(--primary); }

@media (max-width: 640px) {
  .app-header { padding: 12px 14px; }
  .app-header h1 { font-size: 18px; }
  .mode { gap: 6px; }
  .mode select { padding: 4px 8px; font-size: 13px; border-radius: 8px; }

  .quiz {
    gap: 10px;
    width: min(920px, 96%);
    padding-bottom: 90px;
  }
  .stem { padding: 12px; }
  .stem-label { font-size: 12px; }
  .stem-content { min-height: 100px; }
  .stem-content img { max-height: 160px; }

  .options { gap: 8px; }
  .option { padding: 10px 12px; font-size: 14px; }
  .option img { width: 100%; max-height: 80px; object-fit: contain; display: block; }

  .feedback { padding: 12px; gap: 10px; }
  .answer-card { grid-template-columns: 120px 1fr; gap: 12px; align-items: start; }
  .answer-card img { max-height: 120px; }
  .answer-text { font-size: 14px; }

  .controls { justify-content: space-between; flex-wrap: wrap; gap: 8px; }
  .next { padding: 10px 12px; font-size: 14px; border-radius: 10px; }
}