I really had no idea where to post this, but here seemed to be the best of the bad ideas.
I just answered a support query where the user had changed their style to Generator which has its own CSS font/size specifications and thus lost the ability to override the font with CSS in GLOBAL_HEAD.
The solution to this is to remove all font specifications from the system styles, do not set a size on 'normal' text (paragraphs, table cells, body) and where it is necessary to make a given bit of text (such as a heading) larger than the surrounding text using CSS, use relative font sizing with the em or percentage units.
This will ensure that users can achieve maximum customisation and reduce support headaches as people who don't understand CSS at all copy stuff verbatim and then wonder why changing their style breaks it.
Someone who has access to the 
system account needs to do this, of course. Who does, other than Brad?