* {
      box-sizing: border-box;
    }

    body {
      background: #000;
      color: #fff;
      display: flex;
      justify-content: center;
      align-items: center;
      height: 100vh;
      margin: 0;
      font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen;
    }

    .calculator {
      width: 320px;
      background: #000;
      border-radius: 40px;
      padding: 20px;
      box-shadow: 0 10px 40px rgba(0, 0, 0, 0.5);
    }

    .display {
      background: #000;
      color: #fff;
      text-align: right;
      font-size: 48px;
      padding: 20px;
      border-radius: 10px;
      overflow-x: auto;
    }

    .buttons {
      display: grid;
      grid-template-columns: repeat(4, 1fr);
      gap: 10px;
      margin-top: 20px;
    }

    button {
      font-size: 24px;
      border: none;
      border-radius: 50%;
      height: 60px;
      width: 60px;
      cursor: pointer;
      transition: background 0.2s;
    }

    .btn-grey { background: #a5a5a5;
      color: #000;
    }

    .btn-orange {
      background: #ff9500;
      color: #fff;
    }

    .btn-dark {
      background: #333;
      color: #fff;
    }

    .zero {
      grid-column: span 2;
      border-radius: 35px;
      text-align: left;
      padding-left: 24px;
      width: 100%;
    }

    @media (max-width: 400px) {
      .calculator {
        width: 90%;
        border-radius: 30px;
        padding: 15px;
      }

      .display {
        font-size: 36px;
        padding: 15px;
      }

      button {
        height: 50px;
        width: 50px;
        font-size: 20px;
      }
    }