Hi, there is some information about changing fonts over at the-ebook.org. Though it is meant for version 2.x.
Scroll down to "Russification program eBook Library"
http://translate.googleusercontent.c...akfHpjkD6_NODQ
And, to my knowledge this will only affect LRF-Books.
I don't know if Reader Library accepts any userStyle.css for epubs.