body {
      font-family: sans-serif;
      display: flex;
      flex-direction: column;
      align-items: center;
      background: #f0f0f0;
    }
    h1 { margin-top: 20px; }
    #game-board {
      display: grid;
      grid-template-columns: repeat(4, 100px);
      gap: 10px;
      margin: 30px auto;
    }
    .card {
      width: 100px;
      height: 100px;
      background: #4CAF50;
      display: flex;
      align-items: center;
      justify-content: center;
      font-size: 2rem;
      cursor: pointer;
      color: transparent;
      border-radius: 8px;
    }
    .flipped, .matched {
      background: #fff;
      color: black;
      cursor: default;
    }