body {
  background-image: url(https://upload.wikimedia.org/wikipedia/commons/thumb/f/f5/Computer-screen-code-glitch-animation-gif-background-free.gif/960px-Computer-screen-code-glitch-animation-gif-background-free.gif?_=20180213040859);
  background-repeat: repeat; 
  background-position: center;
  background-attachment: fixed;
}