body {
  background-image: url('https://xrystalwish.neocities.org/EnterPageAssets/RainbowBG.gif');
}
  <style>

body {
  background: #000000 url('EnterPageAssets/RainbowBG.gif');
  background-size: cover;
  -webkit-background-size: cover;
  }
  </style>