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

The code was copied from
https://git.wikimedia.org/blob/mediawiki%2Fcore.git/6985d304dbf855562432464d868ad5eae704a416/resources%2Fmediawiki.action%2Fmediawiki.action.history.diff.css
Merge pull request #22 from helder-git/master

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