It doesn't actually change any installed fonts (found in /usr/share/fonts BTW), but rather changes the font definitions in the active hildon-theme to a font of your choice.

So, say you want to use comics-sans as your system font, you can do so as long as the font itself exists in /usr/share/fonts.

You should probably get your head examined if you chose that particular typeface, but that's way off-topic.

MicroB renders fonts as per the configuration. For direct configuration, browse to about:config and have a neb inside there (filter for font for starters), but be warned as per the launching page ... there be dragons.