add css file by javascript code example

Example 1: javascript css link append

document.getElementsByTagName("head")[0].insertAdjacentHTML(
    "beforeend",
    "<link rel=\"stylesheet\" href=\"path/to/style.css\" />");

Example 2: add css style sheet with javascript

<head>

<script>
function myFunction() {
  var x = document.createElement("LINK");
  x.setAttribute("rel", "stylesheet");
  x.setAttribute("type", "text/css");
  x.setAttribute("href", "styles.css");
  document.head.appendChild(x);
}
myFunction()
</script>

</head>