This is depending on which fonts you want to change. Usually, by changing body, h1, h2, h3 is enough for most of the theme. This is because Q2A used those fonts in most of the areas. For example for question heading H1, Widget heading H2 etc.. and body is for the content and other similar areas. However it is depend on how theme is designed but if it is designed with the standard tags than above should work.
So you need to find body fonts in your theme stylesheet (find qa-site/qa-theme/your-theme/qa-styles.css).
So refering Snow Theme
body,td,input,textarea somewhere around on line#70.
Here you will see something similar
font:14px/1.5 Helvetica, Arial, sans-serif;
by chaning font size and family will affect to body, table and form input
Same way find h1 round line#130
Here you can change font-size and font-family and that will affect Question Title and content wrapped with H1 tag
Same for H2 and rest rest of the area.
If you are using any other theme which has different tags or css rule than you may need to find it. You can use firbug or any other inspector to find the tag and rule.