:root {
  --color-primary: #FBBF24;
  --color-primary-dark: #F59E0B;
  --color-success: #10B981;
  --color-error: #EF4444;
  --color-text: #1F2937;
  --color-text-light: #6B7280;
  --color-bg: #FFFFFF;
  --color-bg-alt: #F9FAFB;
  --color-border: #E5E7EB;
  --color-card-shadow: rgba(0, 0, 0, 0.05);
  --font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  --spacing-unit: 8px;
}

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

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

.container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 calc(var(--spacing-unit) * 2);
}

/* Hero Section */
.hero {
  padding: calc(var(--spacing-unit) * 12) 0;
  text-align: center;
  background: var(--color-bg-alt);
}

.hero h1 {
  font-size: 3rem;
  font-weight: 700;
  margin-bottom: calc(var(--spacing-unit) * 3);
  line-height: 1.2;
}

.hero p {
  font-size: 1.25rem;
  color: var(--color-text-light);
  max-width: 600px;
  margin: 0 auto calc(var(--spacing-unit) * 6);
}

/* Stats Grid */
.stats {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
  gap: calc(var(--spacing-unit) * 3);
  margin: calc(var(--spacing-unit) * 8) 0;
}

.stat-card {
  background: var(--color-bg);
  padding: calc(var(--spacing-unit) * 4);
  border-radius: calc(var(--spacing-unit) * 1.5);
  text-align: center;
  box-shadow: 0 2px 8px var(--color-card-shadow);
}

.stat-value {
  font-size: 2.5rem;
  font-weight: 700;
  color: var(--color-primary);
  margin-bottom: calc(var(--spacing-unit) * 1);
}

.stat-label {
  color: var(--color-text-light);
  font-size: 0.875rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

/* Buttons */
.btn {
  display: inline-flex;
  align-items: center;
  padding: calc(var(--spacing-unit) * 2.5) calc(var(--spacing-unit) * 4);
  border-radius: calc(var(--spacing-unit) * 1);
  font-weight: 600;
  text-decoration: none;
  transition: all 0.2s;
  cursor: pointer;
  border: none;
  font-size: 1rem;
}

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

.btn-primary:hover {
  background: var(--color-primary-dark);
  transform: translateY(-2px);
  box-shadow: 0 4px 12px var(--color-card-shadow);
}

.btn-secondary {
  background: var(--color-bg);
  color: var(--color-text);
  border: 2px solid var(--color-border);
}

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

.btn-group {
  display: flex;
  gap: calc(var(--spacing-unit) * 2);
  justify-content: center;
  flex-wrap: wrap;
}

/* Cards */
.card {
  background: var(--color-bg);
  border-radius: calc(var(--spacing-unit) * 1.5);
  padding: calc(var(--spacing-unit) * 4);
  box-shadow: 0 2px 8px var(--color-card-shadow);
  margin-bottom: calc(var(--spacing-unit) * 4);
}

.card/* Footer */
.footer {
  background: var(--color-bg-alt);
  padding: calc(var(--spacing-unit) * 8) 0;
  margin-top: calc(var(--spacing-unit) * 12);
  text-align: center;
}

.footer-tagline {
  font-size: 1.5rem;
  font-weight: 700;
  color: var(--color-primary);
  margin-bottom: calc(var(--spacing-unit) * 2);
}

.footer-description {
  color: var(--color-text-light);
  max-width: 600px;
  margin: 0 auto;
  font-size: 0.875rem;
}

/* Responsive */
@media (max-width: 768px) {
  .hero h1 {
    font-size: 2rem;
  }
  .hero p {
    font-size: 1rem;
  }
  .stats {
    grid-template-columns: repeat(2, 1fr);
  }
}

/* Navigation */
.nav {
  background: var(--color-bg);
  box-shadow: 0 2px 4px var(--color-card-shadow);
  position: sticky;
  top: 0;
  z-index: 100;
}

.nav-content {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: calc(var(--spacing-unit) * 2) 0;
}

.nav-logo {
  font-weight: 700;
  font-size: 1.25rem;
  color: var(--color-primary);
  text-decoration: none;
}

.nav-links {
  display: flex;
  gap: calc(var(--spacing-unit) * 3);
}

.nav-links a {
  color: var(--color-text);
  text-decoration: none;
  font-weight: 500;
  transition: color 0.2s;
}

.nav-links a:hover {
  color: var(--color-primary);
}

/* Flashcard Styles */
.flashcard {
  perspective: 1000px;
  height: 400px;
  cursor: pointer;
}

.flashcard-inner {
  position: relative;
  width: 100%;
  height: 100%;
  text-align: center;
  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;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  padding: calc(var(--spacing-unit) * 4);
  border-radius: calc(var(--spacing-unit) * 1.5);
  box-shadow: 0 4px 12px var(--color-card-shadow);
}

.flashcard-front {
  background: var(--color-bg);
}

.flashcard-back {
  background: var(--color-primary);
  transform: rotateY(180deg);
  color: var(--color-text);
}

/* Quiz Styles */
.quiz-option {
  padding: calc(var(--spacing-unit) * 2);
  border: 2px solid var(--color-border);
  border-radius: calc(var(--spacing-unit) * 1);
  margin-bottom: calc(var(--spacing-unit) * 2);
  cursor: pointer;
  transition: all 0.2s;
}

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

.quiz-option.correct {
  background: var(--color-success);
  border-color: var(--color-success);
  color: white;
}

.quiz-option.incorrect {
  background: var(--color-error);
  border-color: var(--color-error);
  color: white;
}

/* Progress Bar */
.progress-bar {
  width: 100%;
  height: 8px;
  background: var(--color-border);
  border-radius: 4px;
  overflow: hidden;
  margin-bottom: calc(var(--spacing-unit) * 4);
}

.progress-fill {
  height: 100%;
  background: var(--color-primary);
  transition: width 0.3s;
}