I would suspect then the font used for posts is listed in a separate CSS Stylesheet used for it. I've not messed around looking further into XenForo on the styling side of things because I use the default theme and will be sticking with it. But have spotted there are a number of different CSS style-sheets listed under "Templates". Filter them out using the keyword "CSS". Then I would take a look at some of them (perhaps)?
That would be my guess, looking for font styles used in them.