If I access http://django-guardian.readthedocs.org/en/latest/installation.html I see the raw content without any styling. The CSS files appear to load fine, but not having any affect. If I build the documentation manually on my local box it works fine.
I reported the problem as a bug and found that django-guardian provides its own theme that wasn't working. As the custom theme probably wasn't required I have removed it and now it works fine.