body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background: #282c34;
    margin: 0;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
  }
  
  .calculator {
    background: #1e1e1e;
    padding: 20px;
    border-radius: 10px;
    box-shadow: 0 0 15px #00aaffaa;
    width: 320px;
  }
  
  .display {
    width: 100%;
    height: 50px;
    background: #111;
    color: white;
    font-size: 2rem;
    text-align: right;
    border-radius: 6px;
    padding: 10px 0px;
    margin-bottom: 15px;
    border: none;
  }
  
  .buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
  }
  
  .btn {
    background: #333;
    color: white;
    border: none;
    font-size: 1.5rem;
    border-radius: 6px;
    cursor: pointer;
    padding: 15px 0;
    transition: background-color 0.3s ease;
    user-select: none;
  }
  
  .btn:hover {
    background: #00aaff;
  }
  
  .btn.zero {
    grid-column: span 2;
  }
  
  .btn.equals {
    grid-row: span 2;
    background: #00aaff;
  }
  
  .btn.equals:hover {
    background: #008fcc;
  }