body {
  animation: animation-bg-color 10s infinite;
}
@keyframes animation-bg-color {
  0%, 100% {
    background-color: #3498db;
  }
  25% {
    background-color: #1abc9c;
  }
  50% {
    background-color: #9b59b6;
  }
  75% {
    background-color: #e74c3c;
  }
}

External CSS

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External JavaScript

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