:root {
  --bg: #0f172a;
  --card: #1e293b;
  --card-border: #334155;
  --accent: #facc15;
  --accent-ink: #1e293b;
  --text: #f8fafc;
  --muted: #94a3b8;
  --danger: #f87171;
}
* { box-sizing: border-box; }
body {
  margin: 0;
  min-height: 100vh;
  background: linear-gradient(160deg, var(--bg), #1e3a5f);
  color: var(--text);
  font-family: system-ui, "Segoe UI", sans-serif;
}
.hidden { display: none !important; }
.muted { color: var(--muted); }

.screen.center {
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 24px;
}
.card { max-width: 380px; width: 100%; text-align: center; }
.logo { font-size: 48px; margin-bottom: 8px; }
.logo.pulse { animation: pulse 1.4s ease-in-out infinite; }
@keyframes pulse {
  0%, 100% { transform: scale(1); }
  50% { transform: scale(1.15); }
}

/* Phản hồi đúng/sai lúc nộp câu (xem showReveal trong play/app.js). Sai chỉ rung nhẹ
   khung thẻ, KHÔNG đổi màu đỏ/không chữ tiêu cực — tránh cảm giác bị "phạt". */
.card.shake { animation: shake 0.4s; }
@keyframes shake {
  10%, 90% { transform: translateX(-1px); }
  20%, 80% { transform: translateX(2px); }
  30%, 50%, 70% { transform: translateX(-4px); }
  40%, 60% { transform: translateX(4px); }
}

.confetti-piece {
  position: fixed;
  top: -12px;
  width: 8px;
  height: 14px;
  opacity: 0.9;
  pointer-events: none;
  z-index: 50;
  animation-name: confetti-fall;
  animation-timing-function: linear;
  animation-fill-mode: forwards;
}
@keyframes confetti-fall {
  to { transform: translateY(105vh) rotate(540deg); opacity: 0; }
}
h1 { margin: 0 0 8px; }
.subtitle { color: var(--muted); margin: 0 0 24px; }
.link { color: var(--accent); text-decoration: none; }

.field { display: block; margin-bottom: 16px; text-align: left; }
.field span { display: block; font-size: 13px; color: var(--muted); margin-bottom: 6px; }
.field input[type="text"] {
  width: 100%;
  padding: 14px;
  border-radius: 8px;
  border: 1px solid var(--card-border);
  background: var(--card);
  color: var(--text);
  font-size: 18px;
  text-align: center;
  text-transform: uppercase;
  letter-spacing: 2px;
}
#playerNameInput { text-transform: none; letter-spacing: normal; }

.btn {
  border: none;
  border-radius: 10px;
  font-weight: 700;
  cursor: pointer;
  font-family: inherit;
}
.btn-accent { background: var(--accent); color: var(--accent-ink); }
.btn-outline { background: transparent; color: var(--muted); border: 1px solid var(--card-border); }
.btn-large { width: 100%; padding: 16px; font-size: 18px; }
.btn:active { transform: scale(0.98); }
.btn:disabled { opacity: 0.6; }

.skip-btn { margin-top: 16px; font-size: 15px; padding: 12px; }

/* Danh sách số câu — cuộn ngang, để học sinh thấy đã làm câu nào/chưa và bấm quay lại
   làm câu đã bỏ qua (xem play/app.js > renderQuestionNav). */
.q-nav {
  display: flex;
  gap: 8px;
  overflow-x: auto;
  padding: 2px 2px 10px;
  margin-bottom: 8px;
}
.q-nav-chip {
  flex: 0 0 auto;
  width: 34px;
  height: 34px;
  border-radius: 50%;
  border: 1px solid var(--card-border);
  background: var(--card);
  color: var(--text);
  font-weight: 700;
  font-size: 13px;
  font-family: inherit;
  cursor: pointer;
}
.q-nav-chip.current { border-color: var(--accent); background: rgba(250, 204, 21, 0.18); color: var(--accent); }
.q-nav-chip.answered { border-color: transparent; background: var(--card-border); color: var(--muted); opacity: 0.7; cursor: default; }
.q-nav-chip.is-correct { border-color: transparent; background: rgba(74, 222, 128, 0.22); color: #4ade80; opacity: 1; }
.q-nav-chip.is-wrong { border-color: transparent; background: rgba(248, 113, 113, 0.22); color: var(--danger); opacity: 1; }
.q-nav-chip.is-unanswered { border-color: transparent; background: var(--card-border); color: var(--muted); opacity: 0.6; }

.solution-body { background: var(--card); border: 1px solid var(--card-border); border-radius: 12px; padding: 16px; font-size: 15px; line-height: 1.5; }

.error { color: var(--danger); font-size: 14px; margin-top: 12px; }

.screen:not(.center) { padding: 20px; min-height: 100vh; }
.center-text { text-align: center; }

.question-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  margin-bottom: 16px;
}
.q-countdown {
  font-weight: 800;
  color: var(--accent);
  font-variant-numeric: tabular-nums;
}
.q-image {
  display: block;
  max-width: 100%;
  max-height: 260px;
  margin: 0 0 16px;
  border-radius: 10px;
  object-fit: contain;
  background: #fff;
}
.q-text { margin: 0 0 20px; font-size: 20px; }
.q-choices {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 12px;
}
.choice-btn {
  border: 1px solid var(--card-border);
  background: var(--card);
  color: var(--text);
  border-radius: 12px;
  padding: 22px 14px;
  font-size: 16px;
  font-weight: 600;
  font-family: inherit;
  cursor: pointer;
  min-height: 90px;
}
.choice-btn:active { transform: scale(0.97); }
.choice-btn:disabled { opacity: 0.5; }
.choice-btn.selected {
  border-color: var(--accent);
  background: rgba(250, 204, 21, 0.15);
  opacity: 1;
}

@media (max-width: 420px) {
  .q-choices { grid-template-columns: 1fr; }
}

/* Dạng 2: Đúng/Sai 4 ý — #qChoices chuyển sang layout dọc (grid mặc định của
   .q-choices bị ghi đè bởi .tf-group/.short-answer-group). */
.q-choices.tf-group,
.q-choices.short-answer-group {
  display: flex;
  flex-direction: column;
  gap: 12px;
}
.tf-row {
  display: flex;
  align-items: center;
  gap: 12px;
  border: 1px solid var(--card-border);
  background: var(--card);
  border-radius: 12px;
  padding: 14px;
}
.tf-letter { font-weight: 800; color: var(--muted); flex-shrink: 0; }
.tf-text { flex: 1; font-size: 15px; }
.tf-buttons { display: flex; gap: 8px; flex-shrink: 0; }
.tf-choice-btn {
  border: 1px solid var(--card-border);
  background: transparent;
  color: var(--text);
  border-radius: 8px;
  padding: 10px 14px;
  font-size: 14px;
  font-weight: 700;
  font-family: inherit;
  cursor: pointer;
}
.tf-choice-btn:disabled { opacity: 0.5; }
.tf-choice-btn.selected {
  border-color: var(--accent);
  background: rgba(250, 204, 21, 0.15);
}
.tf-submit-btn { margin-top: 4px; }

@media (max-width: 420px) {
  .tf-row { flex-wrap: wrap; }
  .tf-buttons { width: 100%; justify-content: flex-end; }
}

/* Dạng 3: điền đáp án tay */
.short-answer-input {
  width: 100%;
  padding: 18px 14px;
  border-radius: 12px;
  border: 1px solid var(--card-border);
  background: var(--card);
  color: var(--text);
  font-size: 22px;
  font-weight: 700;
  text-align: center;
  font-family: inherit;
}
.short-answer-input:disabled { opacity: 0.5; }