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

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
  min-height: 100vh;
  color: #333;
}

.container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 20px;
}

.header {
  background: rgba(255, 255, 255, 0.95);
  padding: 40px 0;
  text-align: center;
  box-shadow: 0 2px 20px rgba(0, 0, 0, 0.1);
}

.header h1 {
  font-size: 48px;
  color: #667eea;
  margin-bottom: 10px;
}

.subtitle {
  font-size: 20px;
  color: #666;
}

.section {
  padding: 40px 0;
}

.levels-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: 24px;
  margin-top: 30px;
}

.level-card {
  background: white;
  border-radius: 16px;
  padding: 30px;
  cursor: pointer;
  transition: all 0.3s ease;
  box-shadow: 0 4px 15px rgba(0, 0, 0, 0.1);
  position: relative;
  overflow: hidden;
}

.level-card:hover {
  transform: translateY(-8px);
  box-shadow: 0 12px 30px rgba(0, 0, 0, 0.2);
}

.level-badge {
  display: inline-block;
  background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
  color: white;
  font-size: 24px;
  font-weight: bold;
  padding: 8px 16px;
  border-radius: 20px;
  margin-bottom: 15px;
}

.level-card h3 {
  font-size: 24px;
  color: #333;
  margin-bottom: 10px;
}

.level-card p {
  color: #666;
  line-height: 1.6;
}

.hidden {
  display: none;
}

.btn-back {
  background: white;
  border: 2px solid #667eea;
  color: #667eea;
  padding: 10px 20px;
  border-radius: 8px;
  cursor: pointer;
  font-size: 16px;
  margin-bottom: 20px;
  transition: all 0.3s;
}

.btn-back:hover {
  background: #667eea;
  color: white;
}

#level-title {
  font-size: 36px;
  color: white;
  margin-bottom: 30px;
  text-align: center;
}

.tabs {
  display: flex;
  gap: 10px;
  margin-bottom: 30px;
  flex-wrap: wrap;
}

.tab-btn {
  background: rgba(255, 255, 255, 0.9);
  border: none;
  padding: 12px 24px;
  border-radius: 8px;
  cursor: pointer;
  font-size: 16px;
  transition: all 0.3s;
}

.tab-btn:hover {
  background: white;
  transform: translateY(-2px);
}

.tab-btn.active {
  background: white;
  color: #667eea;
  font-weight: bold;
}

.tab-content {
  background: white;
  border-radius: 16px;
  padding: 30px;
  box-shadow: 0 4px 15px rgba(0, 0, 0, 0.1);
  display: none;
}

.tab-content.active {
  display: block;
}

.tab-content h3 {
  font-size: 24px;
  color: #667eea;
  margin-bottom: 20px;
}

.content-grid {
  display: grid;
  gap: 20px;
}

.theory-card {
  background: #f8f9ff;
  border-left: 4px solid #667eea;
  padding: 20px;
  border-radius: 8px;
}

.theory-card h4 {
  color: #333;
  margin-bottom: 10px;
  font-size: 20px;
}

.theory-card p {
  color: #666;
  line-height: 1.6;
  margin-bottom: 10px;
}

.example {
  background: white;
  padding: 15px;
  border-radius: 6px;
  margin-top: 10px;
  font-style: italic;
  color: #555;
}

.vocab-item {
  background: #f8f9ff;
  padding: 15px;
  border-radius: 8px;
  display: flex;
  justify-content: space-between;
  align-items: center;
  transition: all 0.3s;
}

.vocab-item:hover {
  background: #eef0ff;
  transform: translateX(5px);
}

.word-en {
  font-size: 18px;
  font-weight: bold;
  color: #667eea;
}

.word-ru {
  color: #666;
}

.search-input {
  width: 100%;
  padding: 12px 20px;
  border: 2px solid #e0e0e0;
  border-radius: 8px;
  font-size: 16px;
  margin-bottom: 20px;
}

.search-input:focus {
  outline: none;
  border-color: #667eea;
}

.exercise-card {
  background: #f8f9ff;
  padding: 20px;
  border-radius: 8px;
  margin-bottom: 20px;
}

.exercise-question {
  font-size: 18px;
  font-weight: bold;
  color: #333;
  margin-bottom: 15px;
}

.exercise-options {
  display: flex;
  flex-direction: column;
  gap: 10px;
}

.exercise-option {
  padding: 12px 20px;
  background: white;
  border: 2px solid #e0e0e0;
  border-radius: 6px;
  cursor: pointer;
  transition: all 0.3s;
}

.exercise-option:hover {
  border-color: #667eea;
  background: #f0f2ff;
}

.exercise-option.correct {
  border-color: #4caf50;
  background: #e8f5e9;
  color: #2e7d32;
}

.exercise-option.incorrect {
  border-color: #f44336;
  background: #ffebee;
  color: #c62828;
}

.exercise-feedback {
  margin-top: 10px;
  padding: 10px;
  border-radius: 6px;
  font-weight: bold;
}

.exercise-feedback.correct {
  background: #e8f5e9;
  color: #2e7d32;
}

.exercise-feedback.incorrect {
  background: #ffebee;
  color: #c62828;
}

.footer {
  background: rgba(0, 0, 0, 0.2);
  color: white;
  text-align: center;
  padding: 20px 0;
  margin-top: 40px;
}

@media (max-width: 768px) {
  .header h1 {
    font-size: 32px;
  }
  
  .levels-grid {
    grid-template-columns: 1fr;
  }
  
  .tabs {
    flex-direction: column;
  }
  
  .tab-btn {
    width: 100%;
  }
}