body {
}

.css-selector {
  background: linear-gradient(270deg, #ff0000, #000000, #ff0000, #f0b520);
  background-size: 800% 800%;
  -webkit-animation: AnimationName 30s ease infinite;
  -moz-animation: AnimationName 30s ease infinite;
  animation: AnimationName 30s ease infinite;
}

@keyframes AnimationName {
  0% {
    background-position: 0% 50%;
  }
  50% {
    background-position: 100% 50%;
  }
  100% {
    background-position: 0% 50%;
  }
}