It's a while since I played with this, but I think the simple answer is no, FBreader ignores css styling.
The best I was able to do was play with the file called style.xml (I think) in /system/config
You can control the appearance of some epub html tags with this file e.g.
Headers <h1> - <h6>, called "Header 1" to "Header 6"
Paragraph <p>, called "Regular Paragraph"
Subscript, Superscript <sub>, <sup>
Image <img>
The problem is that (hardly surprising given it's called fbreader) it is geared to fb2 files not epub. Most of the fb2 tags are not used in epub.
So, you can control your headers and body text with this style file. But for things like blockquotes, scene-breaks, verse it's pretty unsatisfactory if you're used to epubs displaying correctly, as they do in AdobeViewer. I gave up on fbreader some time ago for this very reason.