body {
    margin: 0;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background-color: hsl(0, 0%, 95%);
    font-size: 16px;
  }
  #calculator {
    font-family: Arial, Helvetica, sans-serif;
    background-color: hsl(0, 0%, 15%);
    border-radius: 20px;
    overflow: hidden;
    max-width: 90vw;
    max-height: 100vh;
    box-shadow: 0 4px 20px rgba(0, 0, 0, 0.2);
  }
  #display {
    width: 100%;
    padding: 16px;
    font-size: 4rem;
    text-align: left;
    border: none;
    background-color: hsl(0, 0%, 20%);
    color: white;
  }
  #keys {
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
    padding: 8px;
  }
  button {
    flex: 1 1 calc(25% - 8px);
    height: 80px;
    border-radius: 10px;
    border: none;
    background-color: hsl(0, 0%, 30%);
    color: white;
    font-size: 3rem;
    font-weight: bold;
    cursor: pointer;
    transition: background-color 0.3s;
    margin: 4px;
  }
  button:hover {
    background-color: hsl(0, 0%, 40%);
  }
  button:active {
    background-color: hsl(0, 0%, 50%);
  }
  .operator-btn {
    background-color: hsl(20, 86%, 50%);
  }
  .operator-btn:hover {
    background-color: hsl(20, 86%, 55%);
  }
  .operator-btn:active {
    background-color: hsl(20, 86%, 70%);
  }
  @media (max-width: 768px) {
    #display {
      font-size: 3.5rem;
    }
    button {
      height: 70px;
      font-size: 2.5rem;
    }
  }
  @media (max-width: 480px) {
    #display {
      font-size: 3rem;
    }
    button {
      height: 60px;
      font-size: 2rem;
    }
  }