body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    margin: 0;
    background: linear-gradient(135deg, #74ebd5, #acb6e5);
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    transition: background 0.4s ease;
  }
  
  body.dark {
    background: linear-gradient(135deg, #232526, #414345);
  }
  
  .theme-toggle {
    position: absolute;
    top: 20px;
    right: 20px;
    cursor: pointer;
    font-size: 24px;
  }
  
  .calculator {
    background: linear-gradient(145deg, #ffffff, #f0f0f0);
    border-radius: 20px;
    box-shadow: 0 10px 25px rgba(0, 0, 0, 0.2);
    padding: 25px;
    width: 330px;
    transition: background 0.3s ease, color 0.3s ease;
  }
  
  body.dark .calculator {
    background: #2b2b2b;
    color: #f9f9f9;
  }
  
  #display {
    width: 100%;
    height: 60px;
    font-size: 26px;
    text-align: right;
    margin-bottom: 20px;
    padding: 15px;
    border: none;
    border-radius: 12px;
    background-color: #f9f9f9;
    color: #333;
    box-shadow: inset 0 2px 5px rgba(0, 0, 0, 0.1);
  }
  
  body.dark #display {
    background-color: #444;
    color: #fff;
  }
  
  .buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 12px;
  }
  
  button {
    padding: 18px;
    font-size: 20px;
    font-weight: bold;
    border: none;
    border-radius: 14px;
    cursor: pointer;
    background-color: #ffffff;
    color: #333;
    box-shadow: 0 4px 10px rgba(0, 0, 0, 0.1);
    transition: 0.2s ease;
  }
  
  button:hover {
    background-color: #e6f0ff;
    transform: scale(1.05);
  }
  
  button:active {
    transform: scale(0.95);
  }
  
  .operator {
    background-color: #ffd166;
  }
  
  .operator:hover {
    background-color: #ffca3a;
  }
  
  .equal {
    background-color: #06d6a0;
    color: white;
  }
  
  .equal:hover {
    background-color: #05c393;
  }
  
  .zero {
    grid-column: span 2;
  }