:root {
  /* Light Mode Variables */
  --bg-color: #f3f4f6;
  --bg-card: #ffffff;
  --bg-panel: #ffffff;
  --text-main: #1f2937;
  --text-muted: #6b7280;
  --border-color: #e5e7eb;
  
  --primary: #4f46e5;
  --primary-hover: #4338ca;
  --secondary: #e5e7eb;
  --secondary-hover: #d1d5db;
  
  --again: #ef4444;
  --hard: #f59e0b;
  --good: #10b981;
  --easy: #3b82f6;

  --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.05);
  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -2px rgba(0, 0, 0, 0.05);
  
  --radius: 12px;
  --radius-sm: 8px;
}

.dark-mode {
  /* Dark Mode Variables */
  --bg-color: #111827;
  --bg-card: #1f2937;
  --bg-panel: #1f2937;
  --text-main: #f9fafb;
  --text-muted: #9ca3af;
  --border-color: #374151;
  
  --primary: #6366f1;
  --primary-hover: #4f46e5;
  --secondary: #374151;
  --secondary-hover: #4b5563;
  
  --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.5);
  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.5);
  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.5);
}

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  font-family: 'Inter', -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  background-color: var(--bg-color);
  color: var(--text-main);
  line-height: 1.5;
  transition: background-color 0.3s, color 0.3s;
  overflow-x: hidden;
}

#app {
  max-width: 800px;
  margin: 0 auto;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

/* Header */
.app-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 1rem 1.5rem;
  background-color: var(--bg-card);
  border-bottom: 1px solid var(--border-color);
  position: sticky;
  top: 0;
  z-index: 10;
}

.logo {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  color: var(--primary);
}

.logo h1 {
  font-size: 1.25rem;
  font-weight: 700;
  color: var(--text-main);
}

/* Layout Helpers */
.hidden { display: none !important; }
.mt-8 { margin-top: 2rem; }
.mb-4 { margin-bottom: 1rem; }
.mb-6 { margin-bottom: 1.5rem; }
.flex-align { display: flex; align-items: center; gap: 0.5rem; }
.flex-align-between { display: flex; justify-content: space-between; align-items: center; }

main {
  flex: 1;
  padding: 1.5rem;
}

/* Views */
.view {
  animation: fadeIn 0.3s ease;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(10px); }
  to { opacity: 1; transform: translateY(0); }
}

.view-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 1.5rem;
}

/* Typography */
h2 { font-size: 1.5rem; font-weight: 600; }
h3 { font-size: 1.1rem; font-weight: 600; }
p { color: var(--text-muted); }

/* Buttons */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 0.5rem;
  padding: 0.6rem 1.2rem;
  border-radius: var(--radius-sm);
  font-weight: 500;
  font-size: 0.95rem;
  cursor: pointer;
  border: none;
  transition: all 0.2s;
  font-family: inherit;
}

.btn.small {
  padding: 0.4rem 0.8rem;
  font-size: 0.85rem;
}

.btn.primary {
  background-color: var(--primary);
  color: white;
}
.btn.primary:hover { background-color: var(--primary-hover); }

.btn.secondary {
  background-color: var(--secondary);
  color: var(--text-main);
}
.btn.secondary:hover { background-color: var(--secondary-hover); }

.btn.outline {
  background-color: transparent;
  border: 1px solid var(--border-color);
  color: var(--text-main);
}
.btn.outline:hover { background-color: var(--secondary); }

.btn.full-width { width: 100%; padding: 1rem; font-size: 1.1rem; }

.icon-btn {
  background: transparent;
  border: none;
  color: var(--text-muted);
  cursor: pointer;
  padding: 0.5rem;
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
  transition: background-color 0.2s, color 0.2s;
}
.icon-btn:hover {
  background-color: var(--secondary);
  color: var(--text-main);
}

/* Deck Grid */
.deck-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
  gap: 1rem;
}

.deck-card {
  background-color: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius);
  padding: 1.5rem;
  cursor: pointer;
  transition: transform 0.2s, box-shadow 0.2s;
  position: relative;
  overflow: hidden;
}

.deck-card:hover {
  transform: translateY(-2px);
  box-shadow: var(--shadow-md);
}

.deck-title {
  font-size: 1.1rem;
  font-weight: 600;
  margin-bottom: 0.5rem;
}

.deck-info {
  font-size: 0.85rem;
  color: var(--text-muted);
}

.deck-actions {
  position: absolute;
  top: 0.5rem;
  right: 0.5rem;
  display: flex;
  opacity: 0;
  transition: opacity 0.2s;
}

.deck-card:hover .deck-actions {
  opacity: 1;
}

/* Card Panel & Lists */
.card-panel {
  background-color: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius);
  padding: 1.5rem;
}

.card-list {
  display: flex;
  flex-direction: column;
  gap: 0.5rem;
}

.list-item-card {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 1rem;
  background-color: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-sm);
}

.list-item-content {
  flex: 1;
  overflow: hidden;
}

.list-item-front, .list-item-back {
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}
.list-item-back {
  color: var(--text-muted);
  font-size: 0.85rem;
  margin-top: 0.2rem;
}

/* Stats Summary */
.stats-summary {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  text-align: center;
  gap: 1rem;
}

.stat-value {
  display: block;
  font-size: 1.5rem;
  font-weight: 700;
  color: var(--primary);
}

.stat-label {
  font-size: 0.85rem;
  color: var(--text-muted);
}

/* Study View */
.study-progress {
  flex: 1;
  margin: 0 1rem;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 0.3rem;
}
#studyProgressText {
  font-size: 0.85rem;
  font-weight: 500;
}
.progress-bar-bg {
  width: 100%;
  height: 6px;
  background-color: var(--secondary);
  border-radius: 3px;
  overflow: hidden;
}
.progress-bar-fill {
  height: 100%;
  background-color: var(--primary);
  width: 0%;
  transition: width 0.3s ease;
}

.btn-tts {
  position: absolute;
  top: 1rem;
  right: 1rem;
  background: transparent;
  border: none;
  color: var(--text-muted);
  cursor: pointer;
  padding: 0.5rem;
  border-radius: 50%;
  z-index: 5;
}
.btn-tts:hover { color: var(--primary); background: var(--secondary); }

/* Flashcard 3D Flip */
.flashcard-container {
  perspective: 1000px;
  width: 100%;
  height: 60vh;
  min-height: 300px;
  margin-bottom: 2rem;
}

.flashcard {
  width: 100%;
  height: 100%;
  cursor: pointer;
}

.flashcard-inner {
  position: relative;
  width: 100%;
  height: 100%;
  text-align: center;
  transition: transform 0.6s cubic-bezier(0.4, 0.2, 0.2, 1);
  transform-style: preserve-3d;
  box-shadow: var(--shadow-lg);
  border-radius: var(--radius);
}

.flashcard.is-flipped .flashcard-inner {
  transform: rotateX(180deg);
}

.flashcard-front, .flashcard-back {
  position: absolute;
  width: 100%;
  height: 100%;
  backface-visibility: hidden;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  padding: 2rem;
  background-color: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius);
  overflow-y: auto;
}

.flashcard-back {
  transform: rotateX(180deg);
  background-color: var(--bg-panel);
}

.card-content {
  font-size: 1.5rem;
  font-weight: 500;
  word-break: keep-all;
  overflow-wrap: break-word;
  max-width: 100%;
}

.answer-text {
  color: var(--primary);
}

.card-reference {
  margin-top: 1.5rem;
  font-size: 0.95rem;
  padding: 1rem;
  background-color: var(--secondary);
  border-radius: var(--radius-sm);
  width: 100%;
  text-align: left;
}
.card-reference span a {
  color: var(--primary);
  text-decoration: underline;
}

.hint-text {
  position: absolute;
  bottom: 1rem;
  font-size: 0.85rem;
  color: var(--text-muted);
}

/* Study Controls (SRS) */
.study-controls {
  width: 100%;
  max-width: 500px;
  margin: 0 auto;
}

.srs-controls {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 0.5rem;
}

.srs-btn {
  flex-direction: column;
  padding: 0.8rem 0;
  gap: 0.2rem;
  color: white;
  border: none;
}

.srs-btn .label { font-size: 0.95rem; font-weight: 600; }
.srs-btn .desc { font-size: 0.75rem; opacity: 0.9; }

.btn-again { background-color: var(--again); }
.btn-again:hover { background-color: #dc2626; }
.btn-hard { background-color: var(--hard); }
.btn-hard:hover { background-color: #d97706; }
.btn-good { background-color: var(--good); }
.btn-good:hover { background-color: #059669; }
.btn-easy { background-color: var(--easy); }
.btn-easy:hover { background-color: #2563eb; }

/* Modals */
.modal {
  position: fixed;
  inset: 0;
  background-color: rgba(0, 0, 0, 0.5);
  display: flex;
  align-items: center;
  justify-content: center;
  z-index: 50;
  backdrop-filter: blur(2px);
}

.modal-content {
  background-color: var(--bg-card);
  width: 90%;
  max-width: 500px;
  border-radius: var(--radius);
  box-shadow: var(--shadow-lg);
  animation: modalIn 0.3s ease;
}

@keyframes modalIn {
  from { opacity: 0; transform: scale(0.95); }
  to { opacity: 1; transform: scale(1); }
}

.modal-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 1.2rem 1.5rem;
  border-bottom: 1px solid var(--border-color);
}

.modal-body {
  padding: 1.5rem;
}

.form-group {
  margin-bottom: 1.2rem;
}

.form-group label {
  display: block;
  font-weight: 500;
  margin-bottom: 0.5rem;
  font-size: 0.95rem;
}

.form-group input, .form-group textarea {
  width: 100%;
  padding: 0.75rem;
  border: 1px solid var(--border-color);
  border-radius: var(--radius-sm);
  background-color: var(--bg-color);
  color: var(--text-main);
  font-family: inherit;
  font-size: 1rem;
}
.form-group input:focus, .form-group textarea:focus {
  outline: none;
  border-color: var(--primary);
  box-shadow: 0 0 0 2px rgba(79, 70, 229, 0.2);
}

.form-actions {
  display: flex;
  justify-content: flex-end;
  gap: 0.8rem;
  margin-top: 1.5rem;
}

/* Data Actions Area */
.btn-group {
  display: flex;
  gap: 0.5rem;
  flex-wrap: wrap;
  margin-top: 1rem;
}

/* PDF Export Classes */
.pdf-card-container {
  width: 595px; /* A4 width roughly */
  display: flex;
  flex-wrap: wrap;
  padding: 20px;
  background: white;
  color: black;
}
.pdf-card {
  width: calc(50% - 20px);
  margin: 10px;
  border: 2px dashed #ccc;
  padding: 20px;
  box-sizing: border-box;
  text-align: center;
  min-height: 200px;
  display: flex;
  flex-direction: column;
  justify-content: center;
}
.pdf-card-title {
  font-size: 18px;
  font-weight: bold;
  margin-bottom: 10px;
}
.pdf-card-desc {
  font-size: 14px;
}