Theme the "Term reference tree" widget in Bilbio

#5777 

@dancabral I've put the CSS in a Biblio specific CSS, but it should probably move to a more general style file -- which one would you consider to be the best one?

![create_publication___john-2](https://cloud.githubusercontent.com/assets/125707/2654619/5b6b866e-bfd6-11e3-923f-91683d91647e.jpg)