#background {
  position: fixed;
  inset: 0;
  z-index: -1000;
  transition: background-color .6s;
  background-repeat: no-repeat;
  background-size: cover;
  background-position: center;
}
#background >svg {
  width: 100%;
  height: 100%;
  opacity: 1;
  transition: opacity .5s;
}
#background >svg.hidden {
  opacity: 0;
}