I just looked at your site, both the screenshot and the site itself. Oddly enough, the site shows a sans-serif font in FireFox, and a serif font in IE in the areas you've highlighted, which tells me IE is playing css inheritance tricks again.
I would try defining the sans serif font-family on the body element, then removing all mention of font-family everywhere else in the stylesheet. Once you've done that, you can selectively add back in changes for the areas you want to be serif font. This will eventually get you where you want to go. Check in ALL browsers for each step you take, and post to the support board for browser checks in systems/browsers you don't have.
I thought I'd mention that your theme design doesn't hold up very well when a user increases font size in any browser, which is something anyone can do, and which can screw up a design royally. Increased to "largest" in IE and three clicks on "increase" in FF it's a fair bit messy with lots of overlapping. You might want to rethink that a bit.