body {
      background: #f0f0f0;
      display: flex;
      justify-content: center;
      align-items: center;
      min-height: 100vh;
      margin: 0;
      font-family: Arial, sans-serif;
    }

    .calc {
      background: white;
      border: 2px solid #ccc;
      border-radius: 8px;
      padding: 20px;
      width: 260px;
    }

    h2 {
      text-align: center;
      margin: 0 0 14px 0;
      font-size: 16px;
      color: #333;
    }

    #screen {
      width: 100%;
      padding: 10px;
      font-size: 1.6rem;
      text-align: right;
      border: 2px solid #aaa;
      border-radius: 6px;
      margin-bottom: 14px;
      background: #e8ffe8;
      box-sizing: border-box;
    }

    .buttons {
      display: grid;
      grid-template-columns: repeat(4, 1fr);
      gap: 8px;
    }

    button {
      padding: 14px;
      font-size: 1rem;
      border: 2px solid #bbb;
      border-radius: 6px;
      cursor: pointer;
      background: #fff;
    }

    button:hover {
      background: #eee;
    }

    .op {
      background: #ffe5b4;
      font-weight: bold;
    }

    .eq {
      background: #90ee90;
      font-weight: bold;
      grid-column: span 2;
    }

    .ac {
      background: #ffb3b3;
      grid-column: span 2;
    }