/* ===================================
   Design System - CSS Custom Properties
   =================================== */
:root {
  /* Colors - Light Theme (default - GenAI style) */
  --bg-primary: #f8fafc;
  --bg-secondary: #ffffff;
  --bg-card: rgba(255, 255, 255, 0.95);
  --bg-glass: rgba(0, 0, 0, 0.03);
  --bg-glass-hover: rgba(0, 0, 0, 0.06);
  --bg-header: rgba(255, 255, 255, 0.95);
  --bg-nav: rgba(255, 255, 255, 0.98);

  --text-primary: #0f172a;
  --text-secondary: #475569;
  --text-muted: #64748b;

  --accent: #2563eb;
  --accent-glow: rgba(37, 99, 235, 0.2);
  --accent-light: #eff6ff;

  --correct: #10b981;
  --correct-bg: rgba(16, 185, 129, 0.08);
  --correct-border: rgba(16, 185, 129, 0.25);
  --correct-glow: rgba(16, 185, 129, 0.12);

  --incorrect: #ef4444;
  --incorrect-bg: rgba(239, 68, 68, 0.08);
  --incorrect-border: rgba(239, 68, 68, 0.25);
  --incorrect-glow: rgba(239, 68, 68, 0.12);

  --unanswered: #f59e0b;
  --unanswered-bg: rgba(245, 158, 11, 0.08);

  --border: #e2e8f0;
  --border-light: rgba(0, 0, 0, 0.06);

  --option-letter-bg: #f1f5f9;
  --vi-bg: rgba(37, 99, 235, 0.05);
  --vi-text: #2563eb;
  --explanation-bg: rgba(37, 99, 235, 0.04);
  --explanation-border: rgba(37, 99, 235, 0.15);

  --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.05);
  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.05), 0 2px 4px -1px rgba(0, 0, 0, 0.03);
  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.05), 0 4px 6px -2px rgba(0, 0, 0, 0.02);

  --radius-sm: 8px;
  --radius-md: 12px;
  --radius-lg: 16px;
  --radius-xl: 20px;

  --font: 'Inter', system-ui, -apple-system, sans-serif;

  /* Safe area for notch phones */
  --safe-top: env(safe-area-inset-top, 0px);
  --safe-bottom: env(safe-area-inset-bottom, 0px);
}

/* ===================================
   Dark Theme
   =================================== */
[data-theme="dark"] {
  --bg-primary: #0a0e1a;
  --bg-secondary: #111827;
  --bg-card: rgba(17, 24, 39, 0.85);
  --bg-glass: rgba(255, 255, 255, 0.04);
  --bg-glass-hover: rgba(255, 255, 255, 0.08);
  --bg-header: rgba(10, 14, 26, 0.92);
  --bg-nav: rgba(10, 14, 26, 0.95);

  --text-primary: #f1f5f9;
  --text-secondary: #94a3b8;
  --text-muted: #64748b;

  --accent: #6366f1;
  --accent-glow: rgba(99, 102, 241, 0.3);
  --accent-light: #818cf8;

  --correct: #10b981;
  --correct-bg: rgba(16, 185, 129, 0.12);
  --correct-border: rgba(16, 185, 129, 0.3);
  --correct-glow: rgba(16, 185, 129, 0.2);

  --incorrect: #ef4444;
  --incorrect-bg: rgba(239, 68, 68, 0.12);
  --incorrect-border: rgba(239, 68, 68, 0.3);
  --incorrect-glow: rgba(239, 68, 68, 0.2);

  --unanswered: #f59e0b;
  --unanswered-bg: rgba(245, 158, 11, 0.12);

  --border: rgba(255, 255, 255, 0.08);
  --border-light: rgba(255, 255, 255, 0.12);

  --option-letter-bg: rgba(255, 255, 255, 0.06);
  --vi-bg: rgba(99, 102, 241, 0.06);
  --vi-text: #818cf8;
  --explanation-bg: rgba(99, 102, 241, 0.06);
  --explanation-border: rgba(99, 102, 241, 0.15);
}

/* ===================================
   Reset & Base
   =================================== */
*,
*::before,
*::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  font-size: 16px;
  -webkit-text-size-adjust: 100%;
  -webkit-tap-highlight-color: transparent;
}

body {
  font-family: var(--font);
  background: var(--bg-primary);
  color: var(--text-primary);
  min-height: 100dvh;
  overflow-x: hidden;
  line-height: 1.6;
  -webkit-font-smoothing: antialiased;
  transition: background 0.3s ease, color 0.3s ease;
}

/* Ambient background gradient */
body::before {
  content: '';
  position: fixed;
  top: -50%;
  left: -50%;
  width: 200%;
  height: 200%;
  background: radial-gradient(ellipse at 20% 20%, rgba(99, 102, 241, 0.08) 0%, transparent 50%),
    radial-gradient(ellipse at 80% 80%, rgba(16, 185, 129, 0.05) 0%, transparent 50%),
    radial-gradient(ellipse at 50% 50%, rgba(245, 158, 11, 0.03) 0%, transparent 50%);
  z-index: -1;
  pointer-events: none;
}

[data-theme="light"] body::before {
  background: radial-gradient(ellipse at 20% 20%, rgba(79, 70, 229, 0.04) 0%, transparent 50%),
    radial-gradient(ellipse at 80% 80%, rgba(5, 150, 105, 0.03) 0%, transparent 50%);
}

button {
  font-family: inherit;
  cursor: pointer;
  border: none;
  background: none;
  color: inherit;
  font-size: inherit;
}

/* ===================================
   Header
   =================================== */
.app-header {
  position: sticky;
  top: 0;
  z-index: 100;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 12px 16px;
  padding-top: calc(12px + var(--safe-top));
  background: var(--bg-header);
  backdrop-filter: blur(20px);
  -webkit-backdrop-filter: blur(20px);
  border-bottom: 1px solid var(--border);
  transition: background 0.3s ease;
}

.header-left {
  display: flex;
  align-items: center;
  gap: 12px;
}

.app-title {
  display: flex;
  align-items: center;
  gap: 6px;
  font-size: 1.1rem;
  font-weight: 700;
  letter-spacing: -0.02em;
}

.title-icon {
  font-size: 1.2rem;
}

.title-text {
  background: linear-gradient(135deg, var(--accent-light), #a78bfa);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

[data-theme="light"] .title-text {
  background: linear-gradient(135deg, var(--accent), #7c3aed);
  -webkit-background-clip: text;
  background-clip: text;
}

.question-counter {
  font-size: 0.8rem;
  color: var(--text-muted);
  font-weight: 500;
  background: var(--bg-glass);
  padding: 4px 10px;
  border-radius: 20px;
  border: 1px solid var(--border);
}

.header-right {
  display: flex;
  align-items: center;
  gap: 6px;
}

.icon-btn {
  width: 40px;
  height: 40px;
  display: flex;
  align-items: center;
  justify-content: center;
  border-radius: 10px;
  background: var(--bg-glass);
  border: 1px solid var(--border);
  color: var(--text-secondary);
  transition: all 0.2s ease;
  position: relative;
}

.icon-btn:hover,
.icon-btn:active {
  background: var(--bg-glass-hover);
  color: var(--text-primary);
}

.icon-btn.active {
  background: var(--accent);
  color: white;
  border-color: var(--accent);
  box-shadow: 0 0 12px var(--accent-glow);
}

.btn-label {
  font-size: 0.75rem;
  font-weight: 700;
  letter-spacing: 0.05em;
}

/* ===================================
   Score Bar
   =================================== */
.score-bar {
  display: flex;
  align-items: center;
  gap: 16px;
  padding: 8px 16px;
  background: var(--bg-secondary);
  border-bottom: 1px solid var(--border);
  position: relative;
  transition: background 0.3s ease;
}

.score-item {
  display: flex;
  align-items: center;
  gap: 5px;
  font-size: 0.85rem;
  font-weight: 600;
}

.score-item .score-icon {
  font-size: 0.75rem;
  width: 20px;
  height: 20px;
  display: flex;
  align-items: center;
  justify-content: center;
  border-radius: 50%;
}

.score-item.correct .score-icon {
  background: var(--correct-bg);
  color: var(--correct);
}

.score-item.incorrect .score-icon {
  background: var(--incorrect-bg);
  color: var(--incorrect);
}

.score-item.unanswered .score-icon {
  color: var(--text-muted);
}

.score-item.correct .score-value {
  color: var(--correct);
}

.score-item.incorrect .score-value {
  color: var(--incorrect);
}

.score-item.unanswered .score-value {
  color: var(--text-muted);
}

.score-progress {
  flex: 1;
  height: 4px;
  background: var(--bg-glass);
  border-radius: 2px;
  overflow: hidden;
}

.progress-bar {
  height: 100%;
  width: 0%;
  background: linear-gradient(90deg, var(--correct), var(--accent-light));
  border-radius: 2px;
  transition: width 0.5s ease;
}

/* ===================================
   Quiz Container
   =================================== */
.quiz-container {
  padding: 16px;
  padding-bottom: calc(80px + var(--safe-bottom));
  max-width: 800px;
  margin: 0 auto;
}

.question-card {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  padding: 20px;
  backdrop-filter: blur(10px);
  -webkit-backdrop-filter: blur(10px);
  animation: fadeInUp 0.3s ease;
  transition: background 0.3s ease;
  box-shadow: var(--shadow-sm);
}

@keyframes fadeInUp {
  from {
    opacity: 0;
    transform: translateY(12px);
  }

  to {
    opacity: 1;
    transform: translateY(0);
  }
}

.question-header {
  display: flex;
  align-items: center;
  gap: 8px;
  margin-bottom: 16px;
  flex-wrap: wrap;
}

.question-badge {
  background: linear-gradient(135deg, var(--accent), #8b5cf6);
  color: white;
  font-size: 0.75rem;
  font-weight: 700;
  padding: 4px 12px;
  border-radius: 20px;
  letter-spacing: 0.02em;
}

.topic-badge {
  background: var(--bg-glass);
  color: var(--text-muted);
  font-size: 0.7rem;
  font-weight: 600;
  padding: 4px 10px;
  border-radius: 20px;
  border: 1px solid var(--border);
}

.question-text {
  font-size: 0.95rem;
  line-height: 1.7;
  color: var(--text-primary);
  margin-bottom: 12px;
  white-space: pre-wrap;
  word-break: break-word;
}

.question-text-vi {
  font-size: 0.88rem;
  line-height: 1.6;
  color: var(--vi-text);
  margin-bottom: 16px;
  padding: 12px;
  background: var(--vi-bg);
  border-radius: var(--radius-sm);
  border-left: 3px solid var(--accent);
  font-style: italic;
  white-space: pre-wrap;
  word-break: break-word;
}

.question-image {
  margin: 10px 0 16px;
  background: var(--bg-glass);
  border: 1px solid var(--border);
  border-radius: var(--radius-md);
  padding: 8px;
}

.question-image img {
  width: 100%;
  border-radius: 10px;
  display: block;
}

/* ===================================
   Options
   =================================== */
.options-list {
  display: flex;
  flex-direction: column;
  gap: 10px;
}

.option-item {
  display: flex;
  align-items: flex-start;
  gap: 12px;
  padding: 14px 16px;
  background: var(--bg-glass);
  border: 1.5px solid var(--border);
  border-radius: var(--radius-md);
  cursor: pointer;
  transition: all 0.2s ease;
  position: relative;
  min-height: 48px;
  -webkit-tap-highlight-color: transparent;
}

.option-item:hover {
  background: var(--bg-glass-hover);
  border-color: var(--border-light);
}

.option-item:active {
  transform: scale(0.99);
}

.option-letter {
  flex-shrink: 0;
  width: 32px;
  height: 32px;
  display: flex;
  align-items: center;
  justify-content: center;
  border-radius: 50%;
  background: var(--option-letter-bg);
  border: 1.5px solid var(--border-light);
  font-weight: 700;
  font-size: 0.8rem;
  color: var(--text-secondary);
  transition: all 0.2s ease;
}

.option-content {
  flex: 1;
  padding-top: 4px;
}

.option-text {
  font-size: 0.9rem;
  line-height: 1.5;
  color: var(--text-primary);
  white-space: pre-wrap;
  word-break: break-word;
}

.option-text-vi {
  font-size: 0.8rem;
  color: var(--vi-text);
  margin-top: 6px;
  font-style: italic;
  display: none;
}

/* Selected state */
.option-item.selected {
  border-color: var(--accent);
  background: var(--accent-glow);
}

.option-item.selected .option-letter {
  background: var(--accent);
  color: white;
  border-color: var(--accent);
}

/* Correct state */
.option-item.correct-answer {
  border-color: var(--correct);
  background: var(--correct-bg);
  box-shadow: 0 0 16px var(--correct-glow);
}

.option-item.correct-answer .option-letter {
  background: var(--correct);
  color: white;
  border-color: var(--correct);
}

/* Wrong state */
.option-item.wrong-answer {
  border-color: var(--incorrect);
  background: var(--incorrect-bg);
  box-shadow: 0 0 16px var(--incorrect-glow);
}

.option-item.wrong-answer .option-letter {
  background: var(--incorrect);
  color: white;
  border-color: var(--incorrect);
}

/* Disabled after answering */
.option-item.disabled {
  cursor: default;
  opacity: 0.7;
}

.option-item.disabled:hover {
  background: var(--bg-glass);
  border-color: var(--border);
}

.option-item.correct-answer.disabled,
.option-item.wrong-answer.disabled {
  opacity: 1;
}

.option-item.correct-answer.disabled:hover {
  background: var(--correct-bg);
  border-color: var(--correct);
}

.option-item.wrong-answer.disabled:hover {
  background: var(--incorrect-bg);
  border-color: var(--incorrect);
}

/* ===================================
   Explanation Box
   =================================== */
.explanation-box {
  margin-top: 20px;
  padding: 16px;
  background: var(--explanation-bg);
  border: 1px solid var(--explanation-border);
  border-radius: var(--radius-md);
  animation: fadeInUp 0.3s ease;
}

.explanation-title {
  display: flex;
  align-items: center;
  gap: 8px;
  font-size: 0.85rem;
  font-weight: 700;
  color: var(--accent-light);
  margin-bottom: 10px;
}

.explanation-content,
.explanation-body {
  font-size: 0.88rem;
  line-height: 1.7;
  color: var(--text-secondary);
  white-space: pre-wrap;
  word-break: break-word;
  font-family: var(--font);
}


/* ===================================
   Navigation
   =================================== */
.quiz-nav {
  position: fixed;
  bottom: 0;
  left: 0;
  right: 0;
  z-index: 100;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 10px 16px;
  padding-bottom: calc(10px + var(--safe-bottom));
  background: var(--bg-nav);
  backdrop-filter: blur(20px);
  -webkit-backdrop-filter: blur(20px);
  border-top: 1px solid var(--border);
  gap: 8px;
  transition: background 0.3s ease;
}

.nav-btn {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 6px;
  padding: 12px 20px;
  font-size: 0.85rem;
  font-weight: 600;
  border-radius: var(--radius-md);
  background: var(--bg-glass);
  border: 1px solid var(--border);
  color: var(--text-secondary);
  transition: all 0.2s ease;
  min-height: 44px;
  flex: 1;
}

.nav-btn:hover,
.nav-btn:active {
  background: var(--bg-glass-hover);
  color: var(--text-primary);
}

.nav-btn.grid-btn {
  flex: 1.2;
  background: rgba(37, 99, 235, 0.08);
  border-color: rgba(37, 99, 235, 0.15);
  color: var(--accent);
}

.nav-btn.grid-btn:hover {
  background: rgba(37, 99, 235, 0.15);
  border-color: rgba(37, 99, 235, 0.25);
}

[data-theme="dark"] .nav-btn.grid-btn {
  background: rgba(99, 102, 241, 0.1);
  border-color: rgba(99, 102, 241, 0.2);
  color: var(--accent-light);
}

[data-theme="dark"] .nav-btn.grid-btn:hover {
  background: rgba(99, 102, 241, 0.2);
  border-color: rgba(99, 102, 241, 0.35);
}

/* ===================================
   Overlay & Side Panel
   =================================== */
.overlay {
  position: fixed;
  inset: 0;
  background: rgba(0, 0, 0, 0.6);
  z-index: 200;
  opacity: 0;
  visibility: hidden;
  transition: all 0.3s ease;
  backdrop-filter: blur(4px);
}

.overlay.visible {
  opacity: 1;
  visibility: visible;
}

.side-panel {
  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  width: min(340px, 85vw);
  z-index: 300;
  background: var(--bg-secondary);
  border-left: 1px solid var(--border);
  transform: translateX(100%);
  transition: transform 0.3s cubic-bezier(0.16, 1, 0.3, 1), background 0.3s ease;
  display: flex;
  flex-direction: column;
  overflow-y: auto;
}

.side-panel.open {
  transform: translateX(0);
}

.panel-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 16px 20px;
  padding-top: calc(16px + var(--safe-top));
  border-bottom: 1px solid var(--border);
  flex-shrink: 0;
}

.panel-header h2 {
  font-size: 1rem;
  font-weight: 700;
}

.close-btn {
  width: 36px;
  height: 36px;
  display: flex;
  align-items: center;
  justify-content: center;
  border-radius: 50%;
  background: var(--bg-glass);
  border: 1px solid var(--border);
  color: var(--text-secondary);
  font-size: 1rem;
  transition: all 0.2s ease;
}

.close-btn:hover {
  background: var(--incorrect-bg);
  color: var(--incorrect);
}

.panel-content {
  padding: 16px 20px;
  overflow-y: auto;
  flex: 1;
}

.panel-section {
  margin-bottom: 24px;
}

.panel-section h3 {
  font-size: 0.85rem;
  font-weight: 700;
  color: var(--text-secondary);
  margin-bottom: 12px;
  letter-spacing: 0.02em;
}

/* Stats Grid */
.stats-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 10px;
}

.stat-box {
  text-align: center;
  padding: 14px 10px;
  border-radius: var(--radius-md);
  background: var(--bg-glass);
  border: 1px solid var(--border);
}

.stat-number {
  font-size: 1.5rem;
  font-weight: 800;
}

.stat-label {
  font-size: 0.72rem;
  color: var(--text-muted);
  margin-top: 4px;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  font-weight: 600;
}

.stat-correct .stat-number {
  color: var(--correct);
}

.stat-incorrect .stat-number {
  color: var(--incorrect);
}

.stat-unanswered .stat-number {
  color: var(--unanswered);
}

.stat-total .stat-number {
  color: var(--accent);
}

[data-theme="dark"] .stat-total .stat-number {
  color: var(--accent-light);
}

/* Settings */
.setting-row {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 12px 0;
  border-bottom: 1px solid var(--border);
  font-size: 0.88rem;
  gap: 12px;
}

.setting-row label:first-child {
  flex: 1;
  color: var(--text-secondary);
}

/* Toggle Switch */
.switch {
  position: relative;
  display: inline-block;
  width: 48px;
  height: 28px;
  flex-shrink: 0;
}

.switch input {
  opacity: 0;
  width: 0;
  height: 0;
}

.slider {
  position: absolute;
  cursor: pointer;
  inset: 0;
  background: var(--bg-glass);
  border: 1px solid var(--border);
  border-radius: 28px;
  transition: all 0.3s ease;
}

.slider::before {
  content: '';
  position: absolute;
  height: 20px;
  width: 20px;
  left: 3px;
  bottom: 3px;
  background: var(--text-muted);
  border-radius: 50%;
  transition: all 0.3s ease;
}

.switch input:checked+.slider {
  background: var(--accent);
  border-color: var(--accent);
}

.switch input:checked+.slider::before {
  transform: translateX(20px);
  background: white;
}

/* Review Buttons */
.review-buttons {
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.review-btn {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 12px 14px;
  background: var(--bg-glass);
  border: 1px solid var(--border);
  border-radius: var(--radius-md);
  color: var(--text-secondary);
  font-size: 0.85rem;
  font-weight: 500;
  transition: all 0.2s ease;
  min-height: 44px;
}

.review-btn:hover,
.review-btn:active {
  background: var(--bg-glass-hover);
  color: var(--text-primary);
}

.review-icon {
  font-size: 1rem;
}

.review-count {
  margin-left: auto;
  background: var(--bg-glass);
  padding: 2px 10px;
  border-radius: 20px;
  font-size: 0.75rem;
  font-weight: 700;
  color: var(--text-muted);
}

.reset-btn {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 6px;
  padding: 12px 14px;
  background: rgba(239, 68, 68, 0.06);
  border: 1px solid rgba(239, 68, 68, 0.15);
  border-radius: var(--radius-md);
  color: var(--incorrect);
  font-size: 0.85rem;
  font-weight: 600;
  transition: all 0.2s ease;
  min-height: 44px;
}

.reset-btn:hover,
.reset-btn:active {
  background: rgba(239, 68, 68, 0.12);
}

/* ===================================
   Question Grid Modal
   =================================== */
.modal-overlay {
  position: fixed;
  inset: 0;
  z-index: 400;
  background: rgba(0, 0, 0, 0.7);
  backdrop-filter: blur(8px);
  display: flex;
  align-items: flex-end;
  opacity: 0;
  visibility: hidden;
  transition: all 0.3s ease;
}

.modal-overlay.visible {
  opacity: 1;
  visibility: visible;
}

.grid-modal-content {
  width: 100%;
  max-height: 80vh;
  background: var(--bg-secondary);
  border-radius: var(--radius-xl) var(--radius-xl) 0 0;
  display: flex;
  flex-direction: column;
  transform: translateY(100%);
  transition: transform 0.35s cubic-bezier(0.16, 1, 0.3, 1), background 0.3s ease;
}

.modal-overlay.visible .grid-modal-content {
  transform: translateY(0);
}

.grid-modal-header {
  display: flex;
  align-items: center;
  flex-wrap: wrap;
  gap: 10px;
  padding: 18px 20px 14px;
  border-bottom: 1px solid var(--border);
  flex-shrink: 0;
}

.grid-modal-header h2 {
  font-size: 1rem;
  font-weight: 700;
  margin-right: auto;
}

.grid-filter-tabs {
  display: flex;
  gap: 4px;
  order: 3;
  width: 100%;
}

.grid-tab {
  flex: 1;
  padding: 8px 6px;
  font-size: 0.72rem;
  font-weight: 600;
  color: var(--text-muted);
  background: var(--bg-glass);
  border: 1px solid var(--border);
  border-radius: 8px;
  transition: all 0.2s ease;
  text-align: center;
}

.grid-tab.active {
  background: var(--accent);
  color: white;
  border-color: var(--accent);
}

.question-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(44px, 1fr));
  gap: 6px;
  padding: 16px 20px;
  padding-bottom: calc(16px + var(--safe-bottom));
  overflow-y: auto;
  max-height: calc(80vh - 110px);
}

.grid-item {
  aspect-ratio: 1;
  display: flex;
  align-items: center;
  justify-content: center;
  border-radius: 8px;
  font-size: 0.75rem;
  font-weight: 700;
  background: var(--bg-glass);
  border: 1px solid var(--border);
  color: var(--text-muted);
  cursor: pointer;
  transition: all 0.15s ease;
  min-width: 38px;
  min-height: 38px;
}

.grid-item:active {
  transform: scale(0.95);
}

.grid-item.current {
  border-color: var(--accent);
  color: var(--accent-light);
  box-shadow: 0 0 8px var(--accent-glow);
}

.grid-item.answered-correct {
  background: var(--correct-bg);
  border-color: var(--correct-border);
  color: var(--correct);
}

.grid-item.answered-incorrect {
  background: var(--incorrect-bg);
  border-color: var(--incorrect-border);
  color: var(--incorrect);
}

/* ===================================
   Toast Notification
   =================================== */
.toast {
  position: fixed;
  bottom: calc(80px + var(--safe-bottom));
  left: 50%;
  transform: translateX(-50%) translateY(20px);
  padding: 10px 20px;
  background: var(--bg-card);
  border: 1px solid var(--border-light);
  border-radius: var(--radius-md);
  backdrop-filter: blur(20px);
  font-size: 0.85rem;
  font-weight: 500;
  color: var(--text-primary);
  z-index: 500;
  opacity: 0;
  transition: all 0.3s ease;
  pointer-events: none;
  white-space: nowrap;
  box-shadow: var(--shadow-md);
}

.toast.show {
  opacity: 1;
  transform: translateX(-50%) translateY(0);
}

/* ===================================
   Responsive - Samsung A16 (360px CSS width)
   =================================== */
@media (max-width: 420px) {
  .app-title {
    font-size: 0.95rem;
  }

  .question-counter {
    font-size: 0.72rem;
  }

  .score-bar {
    gap: 10px;
    padding: 6px 12px;
  }

  .score-item {
    font-size: 0.78rem;
  }

  .quiz-container {
    padding: 10px;
    padding-bottom: calc(75px + var(--safe-bottom));
  }

  .question-card {
    padding: 14px;
    border-radius: var(--radius-md);
  }

  .question-text {
    font-size: 0.88rem;
  }

  .option-item {
    padding: 10px 12px;
    gap: 10px;
  }

  .option-letter {
    width: 28px;
    height: 28px;
    font-size: 0.75rem;
  }

  .option-text {
    font-size: 0.84rem;
  }

  .nav-btn {
    padding: 10px 12px;
    font-size: 0.8rem;
  }

  .question-grid {
    grid-template-columns: repeat(auto-fill, minmax(40px, 1fr));
    gap: 5px;
  }

  .grid-item {
    min-width: 34px;
    min-height: 34px;
    font-size: 0.7rem;
  }
}

/* Larger screens */
@media (min-width: 768px) {
  .quiz-container {
    padding: 24px;
    padding-bottom: calc(90px + var(--safe-bottom));
  }

  .question-card {
    padding: 28px;
  }

  .question-text {
    font-size: 1rem;
  }

  .option-item {
    padding: 16px 20px;
  }

  .grid-modal-content {
    max-width: 600px;
    margin: auto;
    max-height: 70vh;
    border-radius: var(--radius-xl);
  }

  .modal-overlay {
    align-items: center;
    justify-content: center;
  }
}

/* Print */
@media print {

  .app-header,
  .score-bar,
  .quiz-nav,
  .side-panel,
  .overlay,
  .modal-overlay {
    display: none !important;
  }

  .quiz-container {
    padding: 0;
  }

  body {
    background: white;
    color: black;
  }
}

/* Scrollbar */
::-webkit-scrollbar {
  width: 4px;
}

::-webkit-scrollbar-track {
  background: transparent;
}

::-webkit-scrollbar-thumb {
  background: var(--border-light);
  border-radius: 4px;
}

/* Selection */
::selection {
  background: rgba(99, 102, 241, 0.3);
  color: white;
}

[data-theme="light"] ::selection {
  color: #1e293b;
}
/* Concept Link/Highlight Style */
.concept-link {
    color: var(--primary);
    text-decoration: none;
    border-bottom: 1px dashed var(--primary);
    cursor: pointer;
    font-weight: 500;
    transition: all 0.2s ease;
    padding: 0 2px;
    border-radius: 2px;
}

.concept-link:hover {
    color: var(--primary-hover);
    background-color: var(--accent-light);
    border-bottom-style: solid;
}

/* Concept Popover Card Style */
.concept-popover {
    position: absolute;
    z-index: 1000;
    width: 340px;
    background: rgba(255, 255, 255, 0.95);
    backdrop-filter: blur(12px) saturate(180%);
    -webkit-backdrop-filter: blur(12px) saturate(180%);
    border: 1px solid rgba(226, 232, 240, 0.8);
    border-radius: 12px;
    box-shadow: 0 10px 25px -5px rgba(0, 0, 0, 0.1), 0 8px 16px -6px rgba(0, 0, 0, 0.05);
    padding: 1.25rem;
    font-size: 0.9rem;
    line-height: 1.5;
    color: var(--text-main);
    opacity: 0;
    transform: translateY(10px) scale(0.98);
    transition: opacity 0.2s cubic-bezier(0.16, 1, 0.3, 1), transform 0.2s cubic-bezier(0.16, 1, 0.3, 1);
    pointer-events: none; /* Ignore pointer events when invisible */
}

.concept-popover.visible {
    opacity: 1;
    transform: translateY(0) scale(1);
    pointer-events: auto; /* Enable pointer events when visible */
}

.popover-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 0.75rem;
    padding-bottom: 0.5rem;
    border-bottom: 1px solid var(--border);
}

.popover-header h4 {
    font-size: 0.95rem;
    font-weight: 600;
    color: var(--primary);
    margin: 0;
    padding-right: 8px;
}

.popover-badge {
    background-color: var(--accent-light);
    color: var(--primary);
    font-size: 0.7rem;
    font-weight: 600;
    padding: 0.15rem 0.5rem;
    border-radius: 999px;
    white-space: nowrap;
}

.popover-body {
    color: var(--text-main);
    margin-bottom: 0.75rem;
}

.popover-body p {
    margin-bottom: 0.5rem;
}

.popover-body ul {
    margin-top: 0.5rem;
    padding-left: 1.25rem;
    list-style-type: disc;
}

.popover-body li {
    font-size: 0.825rem;
    color: var(--text-muted);
    margin-bottom: 0.25rem;
}

.popover-footer {
    display: flex;
    justify-content: space-between;
    align-items: center;
    font-size: 0.72rem;
    color: var(--text-muted);
    border-top: 1px dashed var(--border);
    padding-top: 0.5rem;
}

/* Custom styling for formatted explanations */
.explanation-content {
  font-size: 0.9rem;
  line-height: 1.7;
  color: var(--text-secondary);
}

.explanation-paragraph {
  margin-bottom: 12px;
}

.explanation-heading {
  font-size: 1rem;
  font-weight: 600;
  color: var(--text-primary);
  margin-top: 16px;
  margin-bottom: 8px;
}

.explanation-list {
  margin-bottom: 12px;
  padding-left: 20px;
}

.explanation-list li {
  margin-bottom: 6px;
}

.explanation-list li strong {
  color: var(--text-primary);
}

.explanation-content code {
  background: var(--bg-glass-hover);
  padding: 2px 6px;
  border-radius: 4px;
  font-family: monospace;
  font-size: 0.85em;
  border: 1px solid var(--border);
}

/* References section */
.references-box {
  margin-top: 16px;
  padding-top: 16px;
  border-top: 1px dashed var(--border);
}

.references-title {
  font-size: 0.85rem;
  font-weight: 700;
  color: var(--accent-light);
  margin-bottom: 8px;
  display: flex;
  align-items: center;
  gap: 6px;
}

[data-theme="light"] .references-title,
[data-theme="light"] .explanation-title {
  color: var(--accent);
}

.references-list {
  list-style-type: none;
  padding: 0;
  margin: 0;
}

.references-list li {
  margin-bottom: 6px;
  padding-left: 14px;
  position: relative;
}

.references-list li::before {
  content: '•';
  color: var(--accent);
  position: absolute;
  left: 0;
  top: 0;
}

.reference-link {
  font-size: 0.85rem;
  color: var(--accent);
  text-decoration: none;
  transition: color 0.2s ease, text-decoration 0.2s ease;
  word-break: break-all;
}

.reference-link:hover {
  color: var(--accent-light);
  text-decoration: underline;
}

[data-theme="light"] .reference-link:hover {
  color: #1d4ed8; /* darker blue */
}

/* Quiz Selector styles */
.quiz-select {
  background: var(--bg-glass);
  border: 1px solid var(--border);
  color: var(--text-primary);
  font-family: inherit;
  font-size: 0.95rem;
  font-weight: 600;
  padding: 6px 12px;
  border-radius: 8px;
  cursor: pointer;
  outline: none;
  transition: all 0.2s ease;
}

.quiz-select:hover {
  background: var(--bg-glass-hover);
  border-color: var(--text-secondary);
}

.quiz-select option {
  background: var(--bg-card);
  color: var(--text-primary);
}

.home-btn {
  width: auto !important;
  padding: 0 12px;
  display: flex;
  gap: 6px;
  font-weight: 500;
  font-size: 0.9rem;
}

@media (max-width: 600px) {
  .home-btn {
    width: 40px !important;
    padding: 0 !important;
  }
  .home-text {
    display: none;
  }
}

/* Multi-choice Submit Button */
.multi-submit-btn {
  width: 100%;
  margin-top: 15px;
  padding: 12px;
  border-radius: 10px;
  border: 1px solid var(--border);
  background: var(--bg-glass);
  color: var(--text-primary);
  font-weight: 600;
  cursor: pointer;
  transition: all 0.2s ease;
}

.multi-submit-btn:disabled {
  opacity: 0.6;
  cursor: not-allowed;
}

.multi-submit-btn:not(:disabled) {
  background: var(--accent);
  color: white;
  border-color: var(--accent);
  box-shadow: 0 0 12px var(--accent-glow);
}

.multi-submit-btn:not(:disabled):hover {
  background: var(--accent-light);
  transform: translateY(-1px);
}