Update the CSS to match the one used by MediaWiki on diff pages

The code was copied from
[mediawiki/core.git / resources / mediawiki.action / mediawiki.action.history.diff.css](https://git.wikimedia.org/blob/mediawiki%2Fcore.git/6985d304dbf855562432464d868ad5eae704a416/resources%2Fmediawiki.action%2Fmediawiki.action.history.diff.css)

This fixes [bug 55924](https://bugzilla.wikimedia.org/show_bug.cgi?id=55924).