This change unifiess the rendering for <example> and <div class=example>.
The former is rendered as HTML <pre>, the latter is taken over. This
patch does the following:
* Merge <pre> CSS from prettify.js int xmpp.css
* Give <pre> a blue-ish background derived from the caption color
* Make the <pre> border use the caption color
* Change div.example background color from neon yellow to a colder and lighter tone that matches the overall color theme
* Add a border to div.example that fits its background color
* Derive width for div.example from <pre>
P.S: Don't eat yellow snow!
