@Gamezcdr: You can't. (At least on FW 2.x, and on FW 3.x, you can only play with what the UI already offers us [ie. basically telling the system that 'sans' should really be that pretty slab serif that you like instead of Helvetica]). But, basically, unless you know and understand what you're doing, don't bother, and just enjoy your 2 or 3 font choices. You can always switch them through the hack and a quick autoreboot.
@Firedb: As always, the only issues with FW 3.x is that the default FT lib seem to choke up on some fonts on some system (Hence the wonky behaviour, if it persists after a full reboot). You can either try to figure out why it does so and fix it by tweaking the font somehow, or use another font, or just enable any of the three FT override options available in the latest hacks (bci being the one most closely matching FW 3.x default rendering settings). Don't forget to reboot (or autoreboot) your Kindle after switching fonts. (And, yes, Serif_ indeed is the family used in vast majority by the reader, the menus are mostly using Sans_ [cf.
this post]). A change in the FT override setting will take a full restart to be taken into account, though, an autoreboot won't suffice.