body {
  color: #fff;
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
  counter-reset: my-count 5;
  animation: countdown 5s linear infinite;
  
  &::after {
    content: counter(my-count);
    font-size: 30em;
  }
}

@keyframes countdown {
  0% {
    background: hsl(0,100%,20%);
  }
  20% {
    counter-increment: my-count -1;
    background: hsl(90,100%,20%);
  }
  40% {
    counter-increment: my-count -2;
    background: hsl(180,100%,20%);
  }
  60% {
    counter-increment: my-count -3;
    background: hsl(270,100%,20%);
  }
  80% {
    counter-increment: my-count -4;
    background: hsl(360,100%,20%);
  }
  100% {
    background: hsl(0,100%,20%);
  }
}