Fix for default.css overrides the theme style

https://github.com/tomchristie/django-rest-framework/issues/775

Example using http://bootswatch.com/flatly/

![screen shot 2013-05-18 at 13 42 09](https://f.cloud.github.com/assets/1503478/521569/0218295a-bfb9-11e2-89ca-a43ff25355ae.png)