Based on what I've read, the recommendation for web design and online content is to use sans serif fonts. Since this practice is a fairly well accepted standard and based on research that indicates that sans serif fonts are easier to read on screen, I would propose that all of the themes included with a standard Moodle install use sans serif fonts for text in menus and on the course page. Of course, if someone is aware of new research that says differently, please let me know.

In reviewing the standard themes, I found that anomaly, serenity, and brick all have serif fonts. I viewed these themes in both Firefox and Internet Explorer. The magazine theme has a mix of serif and sans serif fonts, but since the text in the menus and links on the course page are sans serif, I think it's probably okay.

On a related note, the font size in the formal white theme is significantly smaller than the font size in other themes. I think it should be sized to match the other themes; at this point, it's really too small to be friendly to a majority of users. Again, viewed this theme in Firefox on Mac and Windows and in IE.