Hehe... don't know why I can't resist questions about fonts, but...
You need to get rid of everything below the body tag that says <font size=1 (2,-1, etc)>, and put most of the font sizes up in the CSS part of the template. Well, you need to get rid of them if you want your template to be CSS and HTML compliant anyway.
I've got a post around here somewhere about CSS and HTML compliancy, with a copy of my template in it. I've since modified the template a little though. I changed the CSS BODY tag to say { font-size: small ;} or maybe x-small, can't remember at the moment.
Basically, you want all the font sizes and colors up in the CSS part of the template. Then you want to remove any hard-coded bgcolor font colors and sizes in the lower part of the template. Then make sure any tag that has a font color also has a background color (even if it's transparent or has an image), and use font sizes like small, x-small, xx-small, medium, large, etc. instead of 12px or 12pt.
I'm not sure where windowbg3 is used exactly in YaBB, I set it up to work with my YaPP blocks though. I even added a .menu tag to customize the menu row up top.
The template is fairly flexible as long as you don't remove the nested tables surrounding the <yabb main> area, or it looks pretty bad. You can however remove the extra tables around the logo, menu area and down below by the copyright (just don't remove the copyright itself).
By playing with the width of the table borders, the bordercolor, and by using two or 4 border colors, make some interesting (and some quite ugly) frame effects around your tables too.

If you see a site you like in the Showcase, right-click on the web page and view their source - you'll see what cool things they did to their templates. You can see mine here:
http://ac2m.computers4gaming.com .
Feel free to view my source too!

Have fun, Moya