* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
  }

  #calculator {
    max-width: 400px;
    margin: 50px auto;
    border: 1px solid #333;
    background-color: #eee;
    border-radius: 5px;
    padding: 10px;
    font-family: sans-serif;
  }

  #display {
    background-color: #222;
    color: #fff;
    padding: 15px 10px;
    margin-bottom: 10px;
    border-radius: 3px;
    text-align: right;
    overflow: hidden;
  }

  output {
    font-size: 2.5em;
    min-height: 1.2em;
  }

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

  button {
    padding: 15px 0;
    font-size: 1.2em;
    border: none;
    border-radius: 3px;
    cursor: pointer;
    background-color: #fff;
    transition: background-color 0.1s;
  }

  button:hover {
    filter: brightness(96%);
  }

  [data-action] {
    background-color: #f90;
    color: #fff;
  }
  .back {
    display: block;   
    margin: 1.5rem auto 0;       
    text-decoration: none;
    color: #3b82f6;
    font-weight: 500;
    transition: color 0.3s;
    text-align: center;          
  }
  #zero,
  #equals,
  #clear {
    grid-column: span 2;
  }