You have only to put this couple of rows at the beginning of the DEFAULT.ASPX page, after the various IMPORT and REGISTER

I set two font properties, but it's applicable to every CSS element

You can play with very great numbers to see the effect in the entire your forum, and you can change back simply re-coding a couple of lines

.....
// this is only a reference to see where you must put your code
<%@ Register TagPrefix="YAF" Assembly="YAF" Namespace="YAF" %> 

// you simply put an asterisk to include every CSS element
// indeed since "C" stands for Cascade... this propagates the setting to your whole forum
<style>
  * {
        font-family: Tahoma, Verdana, sans-serif !important;
        font-size: 12pt !important;
    }
</style>
.....

You are perfectly right!

Indeed I called it a "trick" and I told to play with it... simply it may something interesting to know