body, html {

  height: 100%;

  margin: 0;

  padding: 0;

}

​

.shift {

  background-image: repeating-linear-gradient(red, yellow, green, cyan, blue, red 50%);

  background-size: 100% 200%;

  animation: scroll linear 1s infinite;

  width: 100%;

  height: 100%;

}

​

@keyframes scroll {

  from {

    background-position: center 0%;

  }

  to {

    background-position: center 100%;

  }

}