body  {
  animation-name: colors;
  animation-duration: 5s;
  animation-timing-function: ease-in-ease-in-out;
  animation-iteration-count: infinite;
}

@keyframes colors {
  0% {
    background: red;
  }
  60% {
    background: yellow;
  }
  100% {
    background: green;
  }
}