I was going to suggest changing the font size via your broswer control but it appears the Regulus overrides browser selection. Strange....
The themes are currently uneditable except for the options that you are given under the Dashboard -> Presentations -> Theme -> Current Theme Options Tab and for the sidebar widgets. We share the themes between blogs on the site. You may want to read the FAQ at the head of these forums as it's covered in there.
If it really gets to you, you may want to switch themes or think about a paid host for your blog where you can edit the theme yourself.
Hope this helps,