Update theme to match Jenkins UI redesign

This modifies the CSS so that it is compatible with the Jenkins UI redesign ([v1.572](http://jenkins-ci.org/changelog#v1.572)).

As you can see in the image below, it looks similar to before, just a few text changes as a result of the new default Jenkins theme:

![theme](http://puu.sh/ayAgY/fdecc32d91.png)