:root {
  /* Material 3 Light Palette (Default) */
  --md-sys-color-primary: #6750A4;
  --md-sys-color-on-primary: #FFFFFF;
  --md-sys-color-primary-container: #EADDFF;
  --md-sys-color-on-primary-container: #21005D;
  --md-sys-color-secondary: #625B71;
  --md-sys-color-on-secondary: #FFFFFF;
  --md-sys-color-secondary-container: #E8DEF8;
  --md-sys-color-on-secondary-container: #1D192B;
  --md-sys-color-tertiary: #7D5260;
  --md-sys-color-on-tertiary: #FFFFFF;
  --md-sys-color-tertiary-container: #FFD8E4;
  --md-sys-color-on-tertiary-container: #31111D;
  --md-sys-color-error: #B3261E;
  --md-sys-color-on-error: #FFFFFF;
  --md-sys-color-background: #FFFBFE;
  --md-sys-color-on-background: #1C1B1F;
  --md-sys-color-surface: #FFFBFE;
  --md-sys-color-on-surface: #1C1B1F;
  --md-sys-color-surface-variant: #E7E0EC;
  --md-sys-color-on-surface-variant: #49454F;
  --md-sys-color-outline: #79747E;

  /* Custom Aesthetic Variables */
  --glass-bg: rgba(255, 255, 255, 0.7);
  --glass-border: rgba(0, 0, 0, 0.1);
  --glass-blur: blur(20px);
  
  --shadow-sm: 0 2px 4px rgba(0,0,0,0.05);
  --shadow-md: 0 4px 8px rgba(0,0,0,0.1);
  --shadow-lg: 0 12px 24px rgba(0,0,0,0.1);
  --shadow-xl: 0 20px 40px rgba(0,0,0,0.15);
  
  --radius-xs: 4px;
  --radius-sm: 8px;
  --radius-md: 12px;
  --radius-lg: 24px;
  --radius-xl: 32px;
  --radius-full: 9999px;

  --font-sans: 'Google Sans', 'Inter', system-ui, -apple-system, sans-serif;
  --font-mono: 'JetBrains Mono', monospace;
  
  --transition-smooth: all 0.4s cubic-bezier(0.4, 0, 0.2, 1);
  --transition-spring: all 0.5s cubic-bezier(0.175, 0.885, 0.32, 1.275);

  /* Font Scale */
  --font-scale: 1;
}

[data-theme="dark"] {
  --md-sys-color-primary: #D0BCFF;
  --md-sys-color-on-primary: #381E72;
  --md-sys-color-primary-container: #4F378B;
  --md-sys-color-on-primary-container: #EADDFF;
  --md-sys-color-secondary: #CCC2DC;
  --md-sys-color-on-secondary: #332D41;
  --md-sys-color-secondary-container: #4A4458;
  --md-sys-color-on-secondary-container: #E8DEF8;
  --md-sys-color-tertiary: #EFB8C8;
  --md-sys-color-on-tertiary: #492532;
  --md-sys-color-tertiary-container: #633B48;
  --md-sys-color-on-tertiary-container: #FFD8E4;
  --md-sys-color-error: #F2B8B5;
  --md-sys-color-on-error: #601410;
  --md-sys-color-background: #1C1B1F;
  --md-sys-color-on-background: #E6E1E5;
  --md-sys-color-surface: #1C1B1F;
  --md-sys-color-on-surface: #E6E1E5;
  --md-sys-color-surface-variant: #49454F;
  --md-sys-color-on-surface-variant: #CAC4D0;
  --md-sys-color-outline: #938F99;

  --glass-bg: rgba(28, 27, 31, 0.7);
  --glass-border: rgba(255, 255, 255, 0.1);
}

@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --md-sys-color-primary: #D0BCFF;
    --md-sys-color-on-primary: #381E72;
    --md-sys-color-primary-container: #4F378B;
    --md-sys-color-on-primary-container: #EADDFF;
    --md-sys-color-secondary: #CCC2DC;
    --md-sys-color-on-secondary: #332D41;
    --md-sys-color-secondary-container: #4A4458;
    --md-sys-color-on-secondary-container: #E8DEF8;
    --md-sys-color-tertiary: #EFB8C8;
    --md-sys-color-on-tertiary: #492532;
    --md-sys-color-tertiary-container: #633B48;
    --md-sys-color-on-tertiary-container: #FFD8E4;
    --md-sys-color-error: #F2B8B5;
    --md-sys-color-on-error: #601410;
    --md-sys-color-background: #1C1B1F;
    --md-sys-color-on-background: #E6E1E5;
    --md-sys-color-surface: #1C1B1F;
    --md-sys-color-on-surface: #E6E1E5;
    --md-sys-color-surface-variant: #49454F;
    --md-sys-color-on-surface-variant: #CAC4D0;
    --md-sys-color-outline: #938F99;

    --glass-bg: rgba(28, 27, 31, 0.7);
    --glass-border: rgba(255, 255, 255, 0.1);
  }
}

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  -webkit-tap-highlight-color: transparent;
}

body {
  font-family: var(--font-sans);
  background-color: var(--md-sys-color-background);
  color: var(--md-sys-color-on-background);
  line-height: 1.6;
  min-height: 100vh;
  overflow-x: hidden;
  background-image: 
    radial-gradient(circle at 0% 0%, rgba(103, 80, 164, 0.05) 0%, transparent 50%),
    radial-gradient(circle at 100% 0%, rgba(125, 82, 96, 0.05) 0%, transparent 50%),
    radial-gradient(circle at 50% 100%, rgba(98, 91, 113, 0.05) 0%, transparent 50%);
  background-attachment: fixed;
  font-size: calc(1rem * var(--font-scale));
}

[data-theme="dark"] body {
  background-image: 
    radial-gradient(circle at 0% 0%, rgba(208, 188, 255, 0.05) 0%, transparent 50%),
    radial-gradient(circle at 100% 0%, rgba(239, 184, 200, 0.05) 0%, transparent 50%),
    radial-gradient(circle at 50% 100%, rgba(204, 194, 220, 0.05) 0%, transparent 50%);
}

/* --- Settings FAB & Panel --- */
.back-btn {
  position: fixed;
  top: 24px;
  left: 24px;
  padding: 12px 20px;
  background: var(--glass-bg);
  backdrop-filter: var(--glass-blur);
  -webkit-backdrop-filter: var(--glass-blur);
  border: 1px solid var(--glass-border);
  border-radius: var(--radius-full);
  color: var(--md-sys-color-on-surface);
  text-decoration: none;
  font-weight: 600;
  font-size: 0.9rem;
  display: flex;
  align-items: center;
  gap: 8px;
  z-index: 1001;
  box-shadow: var(--shadow-md);
  transition: var(--transition-spring);
}

.back-btn:hover {
  transform: translateX(4px) scale(1.05);
  background: var(--md-sys-color-primary-container);
  color: var(--md-sys-color-on-primary-container);
  box-shadow: var(--shadow-lg);
}

@media (max-width: 768px) {
  .back-btn {
    top: 16px;
    left: 16px;
    padding: 8px 14px;
    font-size: 0.8rem;
  }
}

.settings-fab {
  position: fixed;
  top: 24px;
  right: 24px;
  width: 56px;
  height: 56px;
  border-radius: 16px;
  background: var(--md-sys-color-primary-container);
  color: var(--md-sys-color-on-primary-container);
  border: none;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  box-shadow: var(--shadow-lg);
  z-index: 1001;
  transition: var(--transition-spring);
}

.settings-fab:hover {
  transform: rotate(90deg) scale(1.1);
  box-shadow: var(--shadow-xl);
}

.settings-panel {
  position: fixed;
  top: 96px;
  right: 24px;
  width: 320px;
  background: var(--glass-bg);
  backdrop-filter: var(--glass-blur);
  -webkit-backdrop-filter: var(--glass-blur);
  border: 1px solid var(--glass-border);
  border-radius: var(--radius-xl);
  box-shadow: var(--shadow-xl);
  z-index: 1001;
  padding: 24px;
  transform: translateX(120%) scale(0.9);
  opacity: 0;
  transition: var(--transition-spring);
  pointer-events: none;
}

.settings-panel.active {
  transform: translateX(0) scale(1);
  opacity: 1;
  pointer-events: auto;
}

.settings-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 24px;
}

.settings-header h3 {
  font-size: 1.25rem;
  font-weight: 700;
}

.close-settings {
  background: transparent;
  border: none;
  color: var(--md-sys-color-on-surface-variant);
  cursor: pointer;
  padding: 4px;
}

.setting-item {
  margin-bottom: 24px;
  display: flex;
  flex-direction: column;
}

.setting-item.row {
  flex-direction: row;
  justify-content: space-between;
  align-items: center;
}

.setting-item span {
  font-size: 0.95rem;
  font-weight: 600;
  margin-bottom: 12px;
  color: var(--md-sys-color-on-surface);
}

.setting-item.row span {
  margin-bottom: 0;
}

.theme-switch {
  display: flex;
  background: var(--md-sys-color-surface-variant);
  padding: 4px;
  border-radius: var(--radius-full);
}

.theme-switch button {
  flex: 1;
  border: none;
  background: transparent;
  padding: 8px;
  border-radius: var(--radius-full);
  cursor: pointer;
  color: var(--md-sys-color-on-surface-variant);
  transition: var(--transition-smooth);
}

.theme-switch button.active {
  background: var(--md-sys-color-primary);
  color: var(--md-sys-color-on-primary);
}

.font-size-control {
  display: flex;
  align-items: center;
  justify-content: space-between;
  background: var(--md-sys-color-surface-variant);
  padding: 8px 16px;
  border-radius: var(--radius-full);
}

.font-size-control button {
  background: transparent;
  border: none;
  cursor: pointer;
  color: var(--md-sys-color-primary);
}

/* Switch Styles */
.switch {
  position: relative;
  display: inline-block;
  width: 52px;
  height: 32px;
}

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

.slider {
  position: absolute;
  cursor: pointer;
  top: 0; left: 0; right: 0; bottom: 0;
  background-color: var(--md-sys-color-surface-variant);
  transition: .4s;
  border-radius: 34px;
}

.slider:before {
  position: absolute;
  content: "";
  height: 24px; width: 24px;
  left: 4px; bottom: 4px;
  background-color: white;
  transition: .4s;
  border-radius: 50%;
}

input:checked + .slider { background-color: var(--md-sys-color-primary); }
input:checked + .slider:before { transform: translateX(20px); }

/* --- High-end Animations --- */
.quiz-question.shrunk {
  font-size: 1.1rem;
  opacity: 0.7;
  transform: scale(0.95);
  margin-bottom: 16px;
  transition: var(--transition-smooth);
}

@keyframes shake {
  0%, 100% { transform: translateX(0); }
  20%, 60% { transform: translateX(-10px); }
  40%, 80% { transform: translateX(10px); }
}

.shake {
  animation: shake 0.4s cubic-bezier(.36,.07,.19,.97) both;
}

.correct-emoji {
  position: absolute;
  top: -40px;
  left: 50%;
  transform: translateX(-50%);
  font-size: 2rem;
  animation: bounceIn 0.6s cubic-bezier(0.175, 0.885, 0.32, 1.275);
}

@keyframes bounceIn {
  0% { transform: translateX(-50%) scale(0) translateY(20px); opacity: 0; }
  100% { transform: translateX(-50%) scale(1) translateY(0); opacity: 1; }
}

/* --- Celebration Overlay --- */
.celebration-overlay {
  position: fixed;
  inset: 0;
  background: rgba(0, 0, 0, 0.8);
  backdrop-filter: blur(10px);
  z-index: 2000;
  display: flex;
  align-items: center;
  justify-content: center;
  opacity: 0;
  pointer-events: none;
  transition: opacity 0.5s ease;
}

.celebration-overlay.active {
  opacity: 1;
  pointer-events: auto;
}

.celebration-content {
  background: var(--md-sys-color-surface);
  padding: 48px;
  border-radius: var(--radius-xl);
  text-align: center;
  max-width: 500px;
  width: calc(100% - 48px);
  transform: scale(0.8);
  transition: transform 0.6s cubic-bezier(0.34, 1.56, 0.64, 1);
}

.celebration-overlay.active .celebration-content {
  transform: scale(1);
}

.celebration-icon {
  font-size: 5rem;
  margin-bottom: 24px;
}

.celebration-stats {
  display: flex;
  justify-content: center;
  gap: 40px;
  margin: 32px 0;
}

.stat {
  display: flex;
  flex-direction: column;
}

.stat-value {
  font-size: 2.5rem;
  font-weight: 800;
  color: var(--md-sys-color-primary);
}

.stat-label {
  font-size: 0.9rem;
  color: var(--md-sys-color-on-surface-variant);
}

@media (max-width: 768px) {
  .settings-panel {
    width: calc(100% - 48px);
    right: 24px;
    left: 24px;
  }
}

/* --- Google-level Background Effects --- */

.particles {
  position: fixed;
  inset: 0;
  z-index: -1;
  pointer-events: none;
}

.particle {
  position: absolute;
  background: white;
  border-radius: 50%;
  opacity: 0.1;
  filter: blur(1px);
  animation: floatUp 25s infinite linear;
}

@keyframes floatUp {
  0% { transform: translateY(110vh) translateX(0) rotate(0deg); opacity: 0; }
  10% { opacity: 0.2; }
  90% { opacity: 0.2; }
  100% { transform: translateY(-10vh) translateX(100px) rotate(360deg); opacity: 0; }
}

/* Aura blobs for depth */
.aura-1, .aura-2 {
  position: fixed;
  width: 600px;
  height: 600px;
  border-radius: 50%;
  filter: blur(120px);
  z-index: -2;
  opacity: 0.15;
  pointer-events: none;
}

.aura-1 {
  top: -200px;
  left: -200px;
  background: var(--md-sys-color-primary);
  animation: pulseAura 15s infinite alternate;
}

.aura-2 {
  bottom: -200px;
  right: -200px;
  background: var(--md-sys-color-tertiary);
  animation: pulseAura 20s infinite alternate-reverse;
}

@keyframes pulseAura {
  0% { transform: scale(1) translate(0, 0); }
  100% { transform: scale(1.2) translate(100px, 50px); }
}

/* --- Layout --- */

.container {
  max-width: 960px;
  margin: 0 auto;
  padding: 40px 24px 140px;
}

header {
  text-align: center;
  margin-bottom: 60px;
  animation: slideDown 0.8s cubic-bezier(0.2, 0.8, 0.2, 1);
}

@keyframes slideDown {
  from { transform: translateY(-20px); opacity: 0; }
  to { transform: translateY(0); opacity: 1; }
}

header h1 {
  font-size: 3.5rem;
  font-weight: 800;
  letter-spacing: -0.04em;
  margin-bottom: 12px;
  background: linear-gradient(135deg, var(--md-sys-color-primary) 0%, var(--md-sys-color-tertiary) 100%);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  filter: drop-shadow(0 4px 12px rgba(0,0,0,0.1));
}

header p {
  color: var(--md-sys-color-on-surface-variant);
  font-size: 1.1rem;
  font-weight: 500;
  letter-spacing: 0.3em;
  text-transform: uppercase;
  opacity: 0.8;
  margin-top: 8px;
}

/* --- Navigation (Floating Navigation Bar) --- */

nav {
  position: fixed;
  bottom: 32px;
  left: 50%;
  transform: translateX(-50%);
  z-index: 1000;
  background: var(--glass-bg);
  backdrop-filter: var(--glass-blur);
  -webkit-backdrop-filter: var(--glass-blur);
  padding: 8px;
  border-radius: var(--radius-full);
  border: 1px solid var(--glass-border);
  box-shadow: var(--shadow-xl), 0 0 20px rgba(0,0,0,0.5);
  max-width: 95vw;
}

.nav-wrapper {
  display: flex;
  align-items: center;
  gap: 8px;
}

nav ul {
  list-style: none;
  display: flex;
  gap: 4px;
}

.nav-side-controls {
  display: none; /* Hidden by default, shown via .visible class */
  align-items: center;
  gap: 4px;
  padding-left: 8px;
  border-left: 1px solid var(--glass-border);
  margin-left: 4px;
}

.nav-side-controls.visible {
  display: flex;
}

.nav-side-controls button {
  background: transparent;
  border: none;
  color: var(--md-sys-color-on-surface-variant);
  width: 40px;
  height: 40px;
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
  cursor: pointer;
  transition: var(--transition-smooth);
}

.nav-side-controls button:hover {
  background: rgba(255, 255, 255, 0.1);
  color: var(--md-sys-color-on-surface);
}

nav button {
  background: transparent;
  border: none;
  color: var(--md-sys-color-on-surface-variant);
  padding: 12px 24px;
  font-size: 0.95rem;
  font-weight: 600;
  cursor: pointer;
  border-radius: var(--radius-full);
  transition: var(--transition-smooth);
  display: flex;
  align-items: center;
  gap: 8px;
  white-space: nowrap;
}

nav button:hover {
  background: rgba(255, 255, 255, 0.05);
  color: var(--md-sys-color-on-surface);
}

nav button.active {
  background: var(--md-sys-color-primary-container);
  color: var(--md-sys-color-on-primary-container);
  box-shadow: var(--shadow-md);
}

/* --- Sections --- */

.section {
  display: none;
}

.section.active {
  display: block;
  animation: sectionEnter 0.6s cubic-bezier(0.2, 0.8, 0.2, 1) forwards;
}

@keyframes sectionEnter {
  from { opacity: 0; transform: translateY(20px) scale(0.98); }
  to { opacity: 1; transform: translateY(0) scale(1); }
}

/* --- Component: Cards --- */

.card {
  background: var(--md-sys-color-surface-variant);
  background: linear-gradient(145deg, rgba(73, 69, 79, 0.4), rgba(28, 27, 31, 0.4));
  backdrop-filter: var(--glass-blur);
  -webkit-backdrop-filter: var(--glass-blur);
  border: 1px solid var(--glass-border);
  border-radius: var(--radius-xl);
  padding: 40px;
  box-shadow: var(--shadow-lg);
  margin-bottom: 32px;
  transition: var(--transition-smooth);
}

.card:hover {
  transform: translateY(-4px);
  border-color: rgba(255, 255, 255, 0.2);
  box-shadow: var(--shadow-xl);
}

/* --- Home Section --- */

.welcome-card h2 {
  font-size: 3rem;
  margin-bottom: 20px;
  font-weight: 800;
  letter-spacing: -0.02em;
  background: linear-gradient(to right, #fff, #ccc);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
}

.subtitle {
  color: var(--md-sys-color-on-surface-variant);
  font-size: 1.25rem;
  margin-bottom: 60px;
  max-width: 600px;
  margin-left: auto;
  margin-right: auto;
  line-height: 1.6;
}

.feature-grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 24px;
}

.feature-item {
  background: rgba(255, 255, 255, 0.03);
  border: 1px solid var(--glass-border);
  border-radius: var(--radius-lg);
  padding: 32px 24px;
  cursor: pointer;
  transition: var(--transition-spring);
  text-align: center;
}

.feature-item:hover {
  background: rgba(255, 255, 255, 0.06);
  transform: translateY(-10px) scale(1.02);
  border-color: var(--md-sys-color-primary);
}

.feature-icon {
  margin-bottom: 24px;
  display: flex;
  justify-content: center;
}

.feature-icon i {
  filter: drop-shadow(0 0 8px rgba(208, 188, 255, 0.3));
}

.feature-item h3 {
  font-size: 1.4rem;
  margin-bottom: 12px;
  font-weight: 600;
}

.feature-item p {
  color: var(--md-sys-color-on-surface-variant);
  font-size: 0.95rem;
  line-height: 1.5;
}

/* --- Flashcards (New Design) --- */

.flashcard-deck {
  display: flex;
  flex-direction: column;
  gap: 32px;
  align-items: center;
  margin-top: -20px;
}

.deck-tip {
  color: rgba(255, 255, 255, 0.4);
  font-size: 0.95rem;
  letter-spacing: 0.05em;
  text-align: center;
}

.deck-body {
  display: flex;
  align-items: center;
  gap: 40px;
  width: 100%;
  max-width: 1000px;
  justify-content: center;
}

.flashcard-container {
  width: 100%;
  max-width: 500px;
  perspective: 2000px;
}

.flashcard {
  width: 100%;
  min-height: 600px;
  border-radius: 40px;
  cursor: pointer;
  position: relative;
  transform-style: preserve-3d;
  transition: transform 0.8s cubic-bezier(0.34, 1.56, 0.64, 1);
  box-shadow: 0 30px 60px rgba(0,0,0,0.5);
}

.flashcard.flipped {
  transform: rotateY(180deg);
}

.flashcard-front, .flashcard-back {
  position: absolute;
  inset: 0;
  backface-visibility: hidden;
  padding: 60px;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  border-radius: 40px;
  border: 1px solid rgba(255, 255, 255, 0.1);
}

.flashcard-front {
  background: #2b2b2b;
}

.flashcard-back {
  background: linear-gradient(135deg, #333 0%, #222 100%);
  transform: rotateY(180deg);
}

.flashcard-content {
  width: 100%;
}

.flashcard-content h2 {
  color: #fff;
  font-size: 2.8rem;
  font-weight: 700;
  text-align: left;
  line-height: 1.3;
  width: 100%;
}

#flashcard-answer {
  color: #fff;
  font-size: 1.4rem;
  text-align: left;
  line-height: 1.6;
}

.view-answer-hint {
  position: absolute;
  bottom: 40px;
  color: rgba(255, 255, 255, 0.3);
  font-size: 1rem;
}

.nav-arrow {
  width: 64px;
  height: 64px;
  border-radius: 50%;
  border: none;
  background: rgba(255, 255, 255, 0.05);
  color: #fff;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  transition: var(--transition-spring);
}

.nav-arrow:hover {
  background: var(--md-sys-color-primary);
  transform: scale(1.1);
}

.nav-arrow:disabled {
  opacity: 0.1;
  cursor: not-allowed;
}

.deck-footer {
  width: 100%;
  max-width: 800px;
  margin-top: 20px;
}

.footer-controls {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 32px;
}

.footer-btn {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 8px;
  background: transparent;
  border: none;
  color: #fff;
  cursor: pointer;
  font-size: 0.9rem;
  opacity: 0.8;
  transition: 0.3s;
  white-space: nowrap;
}

.footer-btn:hover {
  opacity: 1;
  color: var(--md-sys-color-primary);
}

.deck-progress {
  flex: 1;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
}

#deck-slider {
  width: 100%;
  -webkit-appearance: none;
  height: 4px;
  background: rgba(255, 255, 255, 0.1);
  border-radius: 2px;
  outline: none;
}

#deck-slider::-webkit-slider-thumb {
  -webkit-appearance: none;
  width: 16px;
  height: 16px;
  background: var(--md-sys-color-primary);
  border-radius: 50%;
  cursor: pointer;
  box-shadow: 0 0 10px rgba(0,0,0,0.5);
}

#deck-counter {
  font-size: 0.9rem;
  color: rgba(255, 255, 255, 0.6);
}

  @media (max-width: 768px) {
    .deck-body {
      gap: 12px;
    }
    .nav-arrow {
      display: none; /* Removed from flashcard deck on mobile */
    }
    
    .flashcard {
      min-height: 450px;
    }
    .flashcard-content h2 {
      font-size: 1.8rem;
    }
    .flashcard-front, .flashcard-back {
      padding: 30px;
    }
    .footer-btn span {
      display: none;
    }
    
    /* Nav adjustments for mobile */
    nav {
      width: auto;
      padding: 4px;
    }
    .nav-wrapper {
      gap: 2px;
    }
    nav button {
      padding: 10px;
    }
    .nav-side-controls {
      padding-left: 4px;
    }
    .nav-side-controls button {
      width: 36px;
      height: 36px;
    }
  }

/* --- Quiz --- */

.quiz-question {
  font-size: 1.6rem;
  margin-bottom: 32px;
  font-weight: 600;
  color: var(--md-sys-color-on-surface);
  line-height: 1.4;
}

.quiz-option {
  display: flex;
  align-items: center;
  gap: 20px;
  padding: 20px 24px;
  background: rgba(255, 255, 255, 0.03);
  border: 1px solid var(--glass-border);
  border-radius: var(--radius-lg);
  cursor: pointer;
  transition: var(--transition-smooth);
  margin-bottom: 12px;
}

.quiz-option:hover:not(.disabled) {
  background: rgba(255, 255, 255, 0.07);
  transform: translateX(10px);
  border-color: var(--md-sys-color-primary);
}

.quiz-option.correct {
  background: rgba(129, 201, 149, 0.15);
  border-color: #81C795;
}

.quiz-option.wrong {
  background: rgba(242, 184, 181, 0.15);
  border-color: #F2B8B5;
}

.option-letter {
  width: 36px;
  height: 36px;
  background: var(--md-sys-color-primary-container);
  color: var(--md-sys-color-on-primary-container);
  border-radius: 50%;
  font-size: 1.1rem;
  font-weight: 700;
  display: flex;
  align-items: center;
  justify-content: center;
  flex-shrink: 0;
}

/* --- Buttons & Controls --- */

.btn {
  padding: 14px 28px;
  border-radius: var(--radius-full);
  font-size: 1rem;
  font-weight: 600;
  cursor: pointer;
  transition: var(--transition-smooth);
  border: 1px solid var(--glass-border);
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 10px;
  background: transparent;
  color: var(--md-sys-color-on-surface);
}

.btn-primary {
  background: var(--md-sys-color-primary);
  color: var(--md-sys-color-on-primary);
  border: none;
}

.btn-primary:hover {
  background: #EADDFF;
  transform: translateY(-2px);
  box-shadow: 0 8px 16px rgba(208, 188, 255, 0.3);
}

.btn-secondary:hover {
  background: rgba(255, 255, 255, 0.08);
  border-color: var(--md-sys-color-primary);
}

select {
  background: var(--md-sys-color-surface-variant);
  color: var(--md-sys-color-on-surface);
  border: 1px solid var(--glass-border);
  padding: 12px 24px;
  border-radius: var(--radius-full);
  font-size: 1rem;
  font-weight: 500;
  outline: none;
  cursor: pointer;
  transition: var(--transition-smooth);
  appearance: none;
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' fill='none' viewBox='0 0 24 24' stroke='white'%3E%3Cpath stroke-linecap='round' stroke-linejoin='round' stroke-width='2' d='M19 9l-7 7-7-7'%3E%3C/path%3E%3C/svg%3E");
  background-repeat: no-repeat;
  background-position: right 20px center;
  background-size: 16px;
  padding-right: 50px;
}

select:focus {
  border-color: var(--md-sys-color-primary);
  box-shadow: 0 0 0 3px rgba(208, 188, 255, 0.2);
}

/* --- Resources (File Explorer Style) --- */

#resources-container {
  display: flex;
  flex-direction: column;
  gap: 24px;
}

.resource-folder {
  border-radius: var(--radius-lg);
  overflow: hidden;
  background: rgba(255, 255, 255, 0.02);
  border: 1px solid var(--glass-border);
}

.folder-header {
  padding: 16px 24px;
  background: rgba(255, 255, 255, 0.05);
  display: flex;
  align-items: center;
  gap: 12px;
  border-bottom: 1px solid var(--glass-border);
}

.folder-name {
  font-weight: 700;
  font-size: 1.1rem;
  flex: 1;
}

.folder-count {
  font-size: 0.85rem;
  color: var(--md-sys-color-on-surface-variant);
  background: var(--md-sys-color-surface-variant);
  padding: 2px 10px;
  border-radius: var(--radius-full);
}

.folder-content {
  display: flex;
  flex-direction: column;
}

.resource-file {
  display: grid;
  grid-template-columns: 48px 1fr 120px 100px;
  align-items: center;
  padding: 12px 24px;
  transition: var(--transition-smooth);
  border-bottom: 1px solid rgba(255, 255, 255, 0.03);
  cursor: pointer;
}

.resource-file:last-child {
  border-bottom: none;
}

.resource-file:hover {
  background: rgba(255, 255, 255, 0.08);
}

.file-icon {
  font-size: 1.5rem;
  display: flex;
  justify-content: center;
}

.file-name {
  font-weight: 500;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
  padding-right: 20px;
}

.file-size {
  font-size: 0.9rem;
  color: var(--md-sys-color-on-surface-variant);
}

.file-actions {
  display: flex;
  gap: 8px;
  justify-content: flex-end;
}

.btn-icon {
  width: 36px;
  height: 36px;
  border-radius: 50%;
  border: none;
  background: transparent;
  color: var(--md-sys-color-on-surface-variant);
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  transition: var(--transition-smooth);
}

.btn-icon:hover {
  background: var(--md-sys-color-primary-container);
  color: var(--md-sys-color-on-primary-container);
}

@media (max-width: 768px) {
  .resource-file {
    grid-template-columns: 40px 1fr 80px;
  }
  .file-size {
    display: none;
  }
}

.progress-container {
  margin: 24px 0;
}

.progress-bar {
  height: 8px;
  background: rgba(255, 255, 255, 0.05);
  border-radius: var(--radius-full);
  overflow: hidden;
}

.progress-bar-fill {
  height: 100%;
  background: var(--md-sys-color-primary);
  box-shadow: 0 0 10px rgba(208, 188, 255, 0.5);
  transition: width 1s cubic-bezier(0.34, 1.56, 0.64, 1);
}

.progress-text {
  font-size: 0.9rem;
  color: var(--md-sys-color-on-surface-variant);
  margin-top: 12px;
  font-weight: 500;
}

/* --- Mobile Optimization --- */

  @media (max-width: 768px) {
    .container { padding: 12px 12px 120px; }
    
    header { margin-bottom: 24px; }
    header h1 { font-size: 2rem; }
    header p { font-size: 0.8rem; letter-spacing: 0.1em; }
    
    .welcome-card h2 { font-size: 1.6rem; }
    .subtitle { font-size: 1rem; margin-bottom: 24px; }
    
    .feature-grid { grid-template-columns: 1fr; gap: 12px; }
    .feature-item { padding: 20px; }
    .feature-item h3 { font-size: 1.1rem; }
    
    .card { padding: 16px; border-radius: var(--radius-lg); margin-bottom: 16px; }
    
    .flashcard { min-height: 280px; }
    .flashcard-front, .flashcard-back { font-size: 1.1rem; padding: 20px; border-radius: var(--radius-lg); }
    
    .flashcard-controls { grid-template-columns: 1fr 1fr; gap: 12px; }
    #mark-learned { grid-column: span 2; order: -1; }
    
    nav { width: auto; bottom: 20px; padding: 4px; }
    nav button { padding: 10px; font-size: 0.85rem; flex: 1; justify-content: center; }
    nav button span { display: none; } /* Hide text on mobile for cleaner look */
    nav button i { width: 22px; height: 22px; }
    
    .nav-wrapper { gap: 2px; }
    .nav-side-controls { padding-left: 4px; }
    .nav-side-controls button { width: 38px; height: 38px; }

    .quiz-question { font-size: 1.25rem; margin-bottom: 20px; }
    .quiz-option { padding: 12px 16px; gap: 10px; border-radius: var(--radius-md); margin-bottom: 8px; }
    .option-letter { width: 28px; height: 28px; font-size: 0.9rem; }
    .option-text { font-size: 0.95rem; }

    .settings-fab { top: 16px; right: 16px; width: 48px; height: 48px; }
  }

@media (max-width: 480px) {
  header h1 { font-size: 2rem; }
  .welcome-card h2 { font-size: 1.8rem; }
  .flashcard-front, .flashcard-back { font-size: 1.15rem; }
}

/* --- Intro / Onboarding --- */

.intro-overlay {
  position: fixed;
  inset: 0;
  background: rgba(0, 0, 0, 0.7);
  z-index: 2000;
  backdrop-filter: blur(4px);
  transition: opacity 0.5s ease;
}

.intro-spotlight {
  position: fixed;
  z-index: 2001;
  box-shadow: 0 0 0 9999px rgba(0, 0, 0, 0.7), 0 0 20px rgba(255, 255, 255, 0.5);
  border-radius: 12px;
  pointer-events: none;
  transition: all 0.4s cubic-bezier(0.34, 1.56, 0.64, 1);
}

.intro-tooltip {
  position: fixed;
  z-index: 2002;
  width: 300px;
  background: var(--md-sys-color-surface);
  color: var(--md-sys-color-on-surface);
  padding: 24px;
  border-radius: 20px;
  box-shadow: var(--shadow-xl);
  opacity: 0;
  transform: translateY(20px);
  transition: all 0.4s cubic-bezier(0.34, 1.56, 0.64, 1);
  pointer-events: auto;
}

.intro-tooltip.active {
  opacity: 1;
  transform: translateY(0);
}

.intro-tooltip-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 12px;
}

.intro-tooltip-header h3 {
  font-size: 1.2rem;
  font-weight: 700;
  color: var(--md-sys-color-primary);
}

.intro-step-count {
  font-size: 0.8rem;
  background: var(--md-sys-color-primary-container);
  color: var(--md-sys-color-on-primary-container);
  padding: 2px 8px;
  border-radius: 10px;
}

.intro-tooltip p {
  font-size: 0.95rem;
  line-height: 1.5;
  margin-bottom: 20px;
  color: var(--md-sys-color-on-surface-variant);
}

.intro-tooltip-footer {
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.intro-btn-skip {
  background: transparent;
  border: none;
  color: var(--md-sys-color-on-surface-variant);
  cursor: pointer;
  font-size: 0.9rem;
  padding: 8px;
}

.intro-btn-next {
  background: var(--md-sys-color-primary);
  color: var(--md-sys-color-on-primary);
  border: none;
  padding: 8px 20px;
  border-radius: 10px;
  cursor: pointer;
  font-weight: 600;
  transition: var(--transition-smooth);
}

.intro-btn-next:hover {
  transform: scale(1.05);
  box-shadow: 0 4px 12px rgba(103, 80, 164, 0.3);
}

.fade-out {
  opacity: 0 !important;
  pointer-events: none !important;
}