At this time, there isn't a direct option to modify the fonts on the sites (with the exception of the fonts in blog entries)
However, if you have some knowledge of HTML coding you may be able to make some modifications in the larger text fields. Currently, our Programming Department Manager allows small amounts of code to be added to the sites. If you try to add your own, and if it's something that they allow on the sites, it will save for you. However, if it's something that he doesn't allow on the sites, (most iframe, forms, flash, slideshows) then the information that you add won't save. (That's the easiest way to find out if it's something that we currently support.)
Here are some links to sites that give some basic html coding instructions:
http://www.w3schools.com/html/html_intro.asp (I prefer this one)
Any coding that you might attempt can typically be utilized in the text fields of the wedding website. Most coding used in the other fields (page title, sidebar links, website URL, etc.) will not work with our coding presets for those areas.