Fix syntax highlighting not apply correctly in slides

public/js/extra.js

@@ -1,5 +1,4 @@
 require('prismjs/themes/prism.css');
-require('highlight.js/styles/github-gist.css');
 
 var Prism = require('prismjs');
 require('prismjs/components/prism-wiki');

public/js/index.js

@@ -6,6 +6,8 @@ require('../css/extra.css');
 require('../css/slide-preview.css');
 require('../css/site.css');
 
+require('highlight.js/styles/github-gist.css');
+
 var toMarkdown = require('to-markdown');
 
 var saveAs = require('file-saver').saveAs;

public/js/pretty.js

@@ -2,6 +2,8 @@ require('../css/extra.css');
 require('../css/slide-preview.css');
 require('../css/site.css');
 
+require('highlight.js/styles/github-gist.css');
+
 /* other vendors plugin */
 var S = require('string');