Made docs sidebar sticky, changed theme to default

- MDN theme appeared fairly bad for markdown use, and the geometric
background was a bit much. Swapped out to default theme.
- Rough-added stickiness to docs sidebar, will need more work once it
starts to expand possible screen height.

resources/js/services/code.js

@@ -185,7 +185,7 @@ function getMode(suggestion, content) {
  * @returns {*|string}
  */
 function getTheme() {
-    return window.codeTheme || 'mdn-like';
+    return window.codeTheme || 'default';
 }
 
 /**