Initially the style is using google web fonts(Open Sans), you can disable the web fonts completely and put your normal font in the Primary/Secondary Font such as:
Primary font controls the general font of the body, secondary controls headers such as navigation, sidebar headings and forum categories.
Additionally you can easily adjust the forum font size using the two properties given beneath the web fonts. It's broken into two sizes, small and large. Large covers areas such as navigation, sidebar headings and forum headers.