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  </div>

index.html

body {
    background-color: blue;
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    margin: 0;
  }

  div {
    display: flex;
    justify-content: center;
    align-items: center;
    background-color: white;
    height: 85%;
    width: 85%;
    transition: background-color 500ms ease;
  }

  div:hover {
    background-color: blue;
  }

index.css