:root {
  --color-accent: #FBBF24;
  --color-accent-dark: #F59E0B;
  --color-success: #10B981;
  --color-error: #EF4444;
  --color-text: #0F172A;
  --color-text-secondary: #64748B;
  --color-text-tertiary: #94A3B8;
  --color-bg: #FFFFFF;
  --color-bg-secondary: #F8FAFC;
  --color-bg-hover: #F8FAFC;
  --color-border: #E2E8F0;
  --font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  --space-xs: 4px;
  --space-sm: 8px;
  --space-md: 16px;
  --space-lg: 24px;
  --space-xl: 32px;
  --space-2xl: 48px;
  --space-3xl: 64px;
  --space-4xl: 96px;
  --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.02);
  --shadow-md: 0 2px 8px 0 rgba(0, 0, 0, 0.04);
  --shadow-lg: 0 4px 16px 0 rgba(0, 0, 0, 0.06);
  --radius: 8px;
  --radius-lg: 12px;
  --transition: 200ms cubic-bezier(0.4, 0, 0.2, 1);
}

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

body {
  font-family: var(--font-family);
  font-size: 1rem;
  line-height: 1.6;
  color: var(--color-text);
  background: var(--color-bg);
}

.container { max-width: 800px; margin: 0 auto; padding: 0 var(--space-lg); }
.container-wide { max-width: 1200px; margin: 0 auto; padding: 0 var(--space-lg); }

/* Navigation */
.nav {
  border-bottom: 1px solid var(--color-border);
  padding: 16px 0;
  background: white;
  position: sticky;
  top: 0;
  z-index: 100;
}

.nav-container {
  display: flex;
  justify-content: space-between;
  align-items: center;
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 var(--space-lg);
}

.nav-brand {
  font-weight: 600;
  color: var(--color-text);
  font-size: 1.125rem;
  text-decoration: none;
  transition: color 0.2s;
}

.nav-brand:hover { color: var(--color-accent-dark); }

.nav-links { display: flex; gap: var(--space-lg); }

.nav-link {
  color: var(--color-text-secondary);
  font-weight: 500;
  text-decoration: none;
  transition: var(--transition);
}

.nav-link:hover, .nav-link.active { color: var(--color-text); }

/* Hero */
.hero {
  min-height: 80vh;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: var(--space-4xl) 0;
  text-align: center;
}

.hero-title {
  font-size: 4rem;
  font-weight: 700;
  line-height: 1.1;
  letter-spacing: -0.02em;
  margin-bottom: var(--space-lg);
}

.hero-subtitle {
  font-size: 1.25rem;
  color: var(--color-text-secondary);
  margin-bottom: var(--space-2xl);
  max-width: 600px;
  margin-left: auto;
  margin-right: auto;
}

/* Animation Demo */
.demo-container {
  background: var(--color-bg-secondary);
  border-radius: var(--radius-lg);
  padding: var(--space-xl);
  margin: var(--space-xl) 0;
  max-width: 500px;
  margin-left: auto;
  margin-right: auto;
}

.demo-word {
  font-size: 1.5rem;
  font-weight: 600;
  color: var(--color-text);
  margin-bottom: var(--space-sm);
}

.demo-meaning {
  font-size: 1rem;
  color: var(--color-text-secondary);
}

/* Stats */
.stat-grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: var(--space-xl);
  margin: var(--space-4xl) 0;
}

.stat-card {
  text-align: center;
  padding: var(--space-2xl);
}

.stat-value {
  font-size: 3rem;
  font-weight: 700;
  color: var(--color-text);
}

.stat-label {
  font-size: 1rem;
  color: var(--color-text-secondary);
  margin-top: var(--space-sm);
}

/* Buttons */
.btn {
  padding: 12px 24px;
  border-radius: var(--radius);
  font-weight: 600;
  transition: var(--transition);
  cursor: pointer;
  border: none;
  text-decoration: none;
  display: inline-block;
  font-size: 1rem;
}

.btn-primary {
  background: var(--color-accent);
  color: var(--color-text);
}

.btn-primary:hover {
  transform: translateY(-2px);
  box-shadow: 0 4px 12px rgba(251, 191, 36, 0.3);
}

.btn-secondary {
  background: var(--color-bg-secondary);
  color: var(--color-text);
  border: 1px solid var(--color-border);
}

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

.btn-large {
  padding: 16px 32px;
  font-size: 1.125rem;
}

.btn-group { display: flex; gap: var(--space-md); justify-content: center; flex-wrap: wrap; }

/* Cards */
.card {
  background: white;
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-lg);
  transition: var(--transition);
  cursor: pointer;
}

.card:hover {
  border-color: var(--color-accent);
  box-shadow: var(--shadow-md);
}

/* Root cards */
.roots-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: var(--space-lg);
  margin: var(--space-xl) 0;
}

.root-card {
  background: white;
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-lg);
  transition: var(--transition);
  cursor: pointer;
}

.root-card:hover {
  border-color: var(--color-accent);
  box-shadow: var(--shadow-md);
}

.root-card.mastered { border-color: var(--color-success); }

.root-name {
  font-size: 1.125rem;
  font-weight: 600;
  color: var(--color-text);
  margin-bottom: var(--space-xs);
}

.root-meaning {
  font-size: 0.875rem;
  color: var(--color-text-secondary);
  margin-bottom: var(--space-sm);
}

.root-origin {
  display: inline-block;
  background: var(--color-accent);
  color: var(--color-text);
  padding: 4px 12px;
  border-radius: var(--radius);
  font-size: 14px;
}

.mastered-badge {
  display: inline-block;
  background: var(--color-success);
  color: #FFFFFF;
  padding: 2px 8px;
  border-radius: var(--radius);
  font-size: 12px;
  margin-left: var(--space-sm);
}

/* Search */
.search-box {
  width: 100%;
  padding: 12px 16px;
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  font-size: 1rem;
  font-family: var(--font-family);
  margin-bottom: var(--space-lg);
  transition: var(--transition);
}

.search-box:focus {
  outline: none;
  border-color: var(--color-accent);
  box-shadow: 0 0 0 3px rgba(251, 191, 36, 0.2);
}

/* Filter */
.filter-container { display: flex; gap: var(--space-sm); flex-wrap: wrap; margin-bottom: var(--space-lg); }

.filter-btn {
  padding: 8px 16px;
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  background: white;
  cursor: pointer;
  transition: var(--transition);
  font-size: 0.875rem;
}

.filter-btn:hover, .filter-btn.active {
  background: var(--color-accent);
  border-color: var(--color-accent);
  color: var(--color-text);
}

/* Flashcard */
.flashcard-container {
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 400px;
  margin: var(--space-2xl) 0;
}

.flashcard {
  width: 100%;
  max-width: 500px;
  height: 320px;
  perspective: 1000px;
  cursor: pointer;
}

.flashcard-inner {
  width: 100%;
  height: 100%;
  position: relative;
  transition: transform 0.6s;
  transform-style: preserve-3d;
}

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

.flashcard-front, .flashcard-back {
  position: absolute;
  width: 100%;
  height: 100%;
  backface-visibility: hidden;
  border-radius: var(--radius-lg);
  padding: var(--space-2xl);
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  text-align: center;
  border: 1px solid var(--color-border);
  background: white;
}

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

.flashcard-root {
  font-size: 1.75rem;
  font-weight: 700;
  color: var(--color-text);
  margin-bottom: var(--space-md);
}

.flashcard-meaning {
  font-size: 1.125rem;
  color: var(--color-text-secondary);
}

.flashcard-description {
  font-size: 1rem;
  color: var(--color-text-secondary);
  line-height: 1.6;
  text-align: left;
  overflow-y: auto;
  max-height: 200px;
}

/* Controls */
.controls {
  display: flex;
  justify-content: center;
  align-items: center;
  gap: var(--space-lg);
  margin: var(--space-xl) 0;
}

.control-btn {
  padding: 12px 24px;
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  background: white;
  cursor: pointer;
  transition: var(--transition);
  font-size: 1rem;
}

.control-btn:hover { background: var(--color-bg-secondary); }

.progress-text {
  font-size: 0.875rem;
  color: var(--color-text-secondary);
}

/* Learn page */
.learn-card {
  background: white;
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-2xl);
  margin: var(--space-xl) 0;
}

.learn-header {
  display: flex;
  justify-content: space-between;
  align-items: flex-start;
  margin-bottom: var(--space-lg);
}

.learn-title {
  font-size: 1.75rem;
  font-weight: 700;
  color: var(--color-text);
}

.learn-meaning {
  font-size: 1.125rem;
  color: var(--color-text-secondary);
  margin-bottom: var(--space-lg);
}

.learn-description {
  font-size: 1rem;
  color: var(--color-text);
  line-height: 1.8;
  margin-bottom: var(--space-xl);
}

/* Examples */
.examples-section { margin: var(--space-xl) 0; }

.examples-title {
  font-size: 1.25rem;
  font-weight: 600;
  margin-bottom: var(--space-md);
}

.example-item {
  background: var(--color-bg-secondary);
  border-radius: var(--radius);
  padding: var(--space-md);
  margin-bottom: var(--space-md);
}

.example-word {
  font-weight: 600;
  color: var(--color-text);
  margin-bottom: var(--space-xs);
}

.example-meaning {
  font-size: 0.875rem;
  color: var(--color-text-secondary);
  margin-bottom: var(--space-xs);
}

.example-explanation {
  font-size: 0.875rem;
  color: var(--color-text-tertiary);
}

/* Quiz */
.quiz-section {
  background: var(--color-bg-secondary);
  border-radius: var(--radius-lg);
  padding: var(--space-xl);
  margin: var(--space-xl) 0;
}

.quiz-question {
  font-size: 1.125rem;
  font-weight: 600;
  margin-bottom: var(--space-lg);
}

.quiz-options { display: flex; flex-direction: column; gap: var(--space-md); }

.quiz-option {
  padding: 12px 16px;
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  background: white;
  cursor: pointer;
  transition: var(--transition);
  text-align: left;
  font-size: 1rem;
}

.quiz-option:hover { border-color: var(--color-accent); }

.quiz-option.correct {
  background: rgba(16, 185, 129, 0.1);
  border-color: var(--color-success);
}

.quiz-option.wrong {
  background: rgba(239, 68, 68, 0.1);
  border-color: var(--color-error);
}

.quiz-option:disabled { cursor: not-allowed; }

/* Progress page */
.progress-stats {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: var(--space-lg);
  margin: var(--space-2xl) 0;
}

.progress-stat-card {
  background: white;
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-xl);
  text-align: center;
}

.progress-stat-value {
  font-size: 2.5rem;
  font-weight: 700;
  color: var(--color-accent-dark);
}

.progress-stat-label {
  font-size: 0.875rem;
  color: var(--color-text-secondary);
  margin-top: var(--space-xs);
}

.progress-bar-container {
  background: var(--color-bg-secondary);
  border-radius: var(--radius);
  height: 12px;
  margin: var(--space-xl) 0;
  overflow: hidden;
}

.progress-bar {
  background: var(--color-accent);
  height: 100%;
  border-radius: var(--radius);
  transition: width 0.5s ease;
}

/* Feedback Toast */
.feedback-toast {
  position: fixed;
  bottom: 24px;
  left: 50%;
  transform: translateX(-50%);
  padding: 12px 24px;
  border-radius: var(--radius);
  font-weight: 500;
  z-index: 1000;
  animation: slideUp 0.3s ease;
}

.feedback-toast.success {
  background: var(--color-success);
  color: white;
}

.feedback-toast.error {
  background: var(--color-error);
  color: white;
}

@keyframes slideUp {
  from { opacity: 0; transform: translateX(-50%) translateY(20px); }
  to { opacity: 1; transform: translateX(-50%) translateY(0); }
}

/* Footer */
.footer {
  background: var(--color-bg-secondary);
  padding: var(--space-4xl) 0;
  text-align: center;
  margin-top: var(--space-4xl);
}

.footer-tagline {
  font-size: 1.5rem;
  font-weight: 600;
  color: var(--color-text);
  margin-bottom: var(--space-md);
}

.footer-description {
  font-size: 1rem;
  color: var(--color-text-secondary);
  max-width: 600px;
  margin: 0 auto;
  line-height: 1.6;
}

/* Section title */
.section-title {
  font-size: 1.5rem;
  font-weight: 600;
  color: var(--color-text);
  margin: var(--space-2xl) 0 var(--space-lg);
}

/* Detail page */
.detail-header {
  margin-bottom: var(--space-xl);
}

.detail-title {
  font-size: 2rem;
  font-weight: 700;
  color: var(--color-text);
  margin-bottom: var(--space-sm);
}

.detail-origin {
  display: inline-block;
  background: var(--color-accent);
  color: var(--color-text);
  padding: 4px 12px;
  border-radius: var(--radius);
  font-size: 14px;
}

/* Responsive */
@media (max-width: 768px) {
  .hero-title { font-size: 2.5rem; }
  .stat-grid { grid-template-columns: 1fr; gap: var(--space-lg); }
  .progress-stats { grid-template-columns: 1fr; }
  .roots-grid { grid-template-columns: 1fr; }
  .nav-links { gap: var(--space-md); font-size: 0.875rem; }
  .flashcard { height: 280px; }
}

/* Utility */
.text-center { text-align: center; }
.mt-2xl { margin-top: var(--space-2xl); }
.mb-2xl { margin-bottom: var(--space-2xl); }