The fonts are defined in the stylesheet of your theme (usually qa-styles.css). In its simplest form a font definition will look somewhat like this:
font-family: Verdana, Arial, Helvetica, sans-serif;
The above tells the browser to render text in the Verdana, Arial or Helvetica font (whichever is found first on the user's computer), and fall back to the generic "sans-serif" that is configured in the user's browser if none of the former are present. Helvetica, Arial and Verdana are the most common sans-serif fonts in the usual operating systems (Windows, Mac OS, Linux), hence font-family attributes for sans-serif text usually consist of those. The equivalent list for serif fonts is something like "Georgia, Garamond, Times New Roman, Roman, serif."
If you want to use a custom font that may not be present on the user's computer you'll have to provide the font file(s) in the Web Open Font Format (WOFF) and define a custom font-face. I posted the details in this answer on my own Q&A site.