PB,
You can set the default font in the preferences. See this page on customizing preferences:
http://kmeleon.sourceforge.net/docs/custompref.php
and then see this page for the particular preference you want to change:
http://kmeleon.sourceforge.net/docs/mozpref.php
You can specify the default font used as well as the font family. Hopefully, we'll get this included in the actual broswer interface in the next version.
Andrew