FBReader does
not properly use CSS. It's built on an FB2 rendering engine, and every other format is forced into it's display properties. All CSS formatting is discarded. This personally drives me crazy because every single other reader program for Linux either displays only text or is built on some gigantic HTML engine, and so cannot be ported to a small device properly.
Micro-culturally, the FBReader guys are married to FBReaderJ and to adding formats. FBReader is now rarely updated, and they seem happy to have extracted bare text from HTML and call it "ePub supported". Meh.
If anyone here can program, a stand-alone ePub reader based on the
dillo rendering engine would be small, fast and awesome. It doesn't support full CSS yet, but it's on its (slow) way.