body, html {
    margin: 0;
    height: 100%;
  }
  
  #caja {
    width: 50vw;
    height: 50vh;
    border: 2px solid black;
    background-color: white;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    transition: background-color 0.2s ease;
  }