I managed to solve it. Below the solution:
Add the code below at the top of the page-header.html file:
<link href="//cdn.bootcss.com/highlight.js/10.1.0/styles/default.min.css" rel="stylesheet">
<script src="//cdn.bootcss.com/highlight.js/10.1.0/highlight.min.js"></script>
<script src="//cdn.bootcss.com/highlight.js/10.1.0/languages/r.min.js"></script>
<script>
hljs.configure({languages: []});
hljs.initHighlightingOnLoad();
</script>
change default to the style of your choice and r change to the language of interest.