Do konsoli:
var styles = `body{animation-name: fexample;animation-duration: 10s;animation-iteration-count: infinite;}@keyframes fexample {0%   {color: red;}40%  {color: yellow;}80% {color: green;}100% {color: RED;}}`;var styleSheet = document.createElement("style");styleSheet.type = "text/css";styleSheet.innerText = styles;document.head.appendChild(styleSheet)

Do custom css:
body{
  animation-name: fexample;
  animation-duration: 10s;
  animation-iteration-count: infinite;
}
@keyframes fexample {
  0%   {color: red;}
  40%  {color: yellow;}
  80% {color: green;}
  100% {color: RED;}
}