body {
  background-image: url(rave.gif);
  height: 98%;
  width: 98%;
  margin: 0;
  padding: 0;
}
.center {
  position: fixed;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  border: 10px solid transparent;
  border-image: url(rainbow.gif) 1 round;
  z-index: -999;
}