match the background of pre with code (the github theme of highlight.js defined the background of code to be #f8f8f8)

exampleSite/layouts/partials/foot_custom.html

@@ -2,3 +2,11 @@
 <script async src="//cdn.bootcss.com/mathjax/2.7.1/MathJax.js?config=TeX-MML-AM_CHTML"></script>
 
 <script async src="//yihui.name/js/center-img.js"></script>
+
+<script src="//cdn.bootcss.com/highlight.js/9.12.0/highlight.min.js"></script>
+<script src="//cdn.bootcss.com/highlight.js/9.12.0/languages/r.min.js"></script>
+
+<script>
+hljs.configure({languages: []});
+hljs.initHighlightingOnLoad();
+</script>

exampleSite/layouts/partials/head_custom.html

@@ -0,0 +1 @@
+<link href="//cdn.bootcss.com/highlight.js/9.12.0/styles/github.min.css" rel="stylesheet">

static/css/style.css

@@ -26,6 +26,7 @@ hr {
 pre {
   border: 1px solid #ddd;
   box-shadow: 5px 5px 5px #eee;
+  background: #f8f8f8;
   padding: 1em;
   overflow-x: auto;
 }