body {
      font-family: Arial, sans-serif;
      padding: 20px;
      background: #f4f4f4;
    }
    .grid {
      display: flex;
      justify-content: space-between;
      flex-wrap: wrap;
      gap: 10px;
    }
    .group {
      background: #fff;
      padding: 10px;
      border-radius: 8px;
      flex: 1;
      min-width: 100px;
      text-align: center;
      box-shadow: 0 0 5px rgba(0,0,0,0.1);
    }
    .header {
      background-color: #003366;
      color: white;
      font-weight: bold;
      padding: 8px;
      border-radius: 5px 5px 0 0;
      border-bottom: 2px solid #ccc;
      transition: transform 0.2s;
    }
    .divider {
      height: 4px;
      background: #ccc;
      margin: 6px 0 10px 0;
    }
    .letter {
      margin: 4px auto;
      padding: 6px;
      border-radius: 5px;
      width: 50px;
      transition: transform 0.2s;
    }
    .empty {
      visibility: hidden;
    }
    .highlight {
      transform: scale(1.25);
      z-index: 10;
    }
    @media (max-width: 768px) {
      .grid {
        flex-direction: column;
        align-items: center;
      }
      .group {
        width: 90%;
      }
    }