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

Author: yihui

static/css/style.css

@@ -25,6 +25,7 @@ hr {
 pre {
   border: 1px solid #ddd;
   box-shadow: 5px 5px 5px #eee;
+  background: #f8f8f8;
   padding: 1em;
   overflow-x: auto;
 }