.header {
        text-align: center;
        margin-bottom: 30px;
        color: white;
      }

      .header h1 {
        font-size: 2.5rem;
        margin-bottom: 10px;
        text-shadow: 2px 2px 4px rgba(0, 0, 0, 0.3);
      }

      .header p {
        font-size: 1.2rem;
        opacity: 0.9;
      }


      .btn {
        padding: 10px 20px;
        background: rgba(255, 255, 255, 0.2);
        color: white;
        border: 2px solid rgba(255, 255, 255, 0.3);
        border-radius: 25px;
        cursor: pointer;
        font-size: 1rem;
        transition: all 0.3s ease;
        backdrop-filter: blur(10px);
      }

      .btn:hover {
        background: rgba(255, 255, 255, 0.3);
        border-color: rgba(255, 255, 255, 0.6);
        transform: translateY(-2px);
      }

      .btn.active {
        background: rgba(255, 255, 255, 0.4);
        border-color: white;
      }

      .vocab-grid {
        display: grid;
        grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
        gap: 20px;
        max-width: 1200px;
        margin: 0 auto;
      }

      .flip-card {
        background-color: transparent;
        width: 100%;
        height: 200px;
        perspective: 1000px;
      }

      .flip-card-inner {
        position: relative;
        width: 100%;
        height: 100%;
        text-align: center;
        transition: transform 0.6s;
        transform-style: preserve-3d;
        cursor: pointer;
      }

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

      .flip-card-front,
      .flip-card-back {
        position: absolute;
        width: 100%;
        height: 100%;
        backface-visibility: hidden;
        border-radius: 15px;
        box-shadow: 0 8px 32px rgba(0, 0, 0, 0.2);
        display: flex;
        flex-direction: column;
        justify-content: center;
        align-items: center;
        padding: 20px;
      }

      .flip-card-front {
        background: linear-gradient(145deg, #ffffff, #f0f0f0);
        color: #333;
      }

      .flip-card-back {
        background: linear-gradient(145deg, #4facfe, #00f2fe);
        color: white;
        transform: rotateY(180deg);
      }

      .word {
        font-size: 1.8rem;
        font-weight: bold;
        margin-bottom: 10px;
        color: #2c3e50;
      }

      .pronunciation {
        font-size: 1rem;
        color: #7f8c8d;
        font-style: italic;
        margin-bottom: 5px;
      }

      .level {
        display: inline-block;
        padding: 4px 12px;
        border-radius: 20px;
        font-size: 0.8rem;
        font-weight: bold;
        text-transform: uppercase;
      }

      .level.beginner {
        background: #2ecc71;
        color: white;
      }

      .level.intermediate {
        background: #f39c12;
        color: white;
      }

      .level.advanced {
        background: #e74c3c;
        color: white;
      }

      .definition {
        font-size: 1.2rem;
        font-weight: bold;
        margin-bottom: 15px;
      }

      .example {
        font-size: 1rem;
        font-style: italic;
        opacity: 0.9;
        text-align: center;
      }

      .flip-hint {
        position: absolute;
        bottom: 10px;
        right: 15px;
        font-size: 0.8rem;
        opacity: 0.6;
      }

      .stats {
        text-align: center;
        margin-bottom: 20px;
        color: white;
        font-size: 1.1rem;
      }

      .category-tag {
        display: inline-block;
        background: #f3f4f6;
        color: #4f46e5;
        font-size: 0.85rem;
        padding: 0.25rem 0.7rem;
        border-radius: 12px;
        margin: 0 0.2rem 0.2rem 0;
        font-weight: 500;
      }

      @media (max-width: 768px) {
        .vocab-grid {
          grid-template-columns: 1fr;
          gap: 15px;
        }

        .header h1 {
          font-size: 2rem;
        }

        .controls {
          flex-direction: column;
          align-items: center;
        }
      }