If you hold down the OK button when opening a book in the library, you will get a menu with various items. One is "Open with", which allows you to choose which of multiple apps should be used to open the book. Next time you open the same book, then the last one you chose will be used to open it. By default, AdobeViewer.app will be picked over fbreader.app when opening new EPUB books. You can change this by creating your own
/system/config/extensions.cfg file and overriding the entry for EPUB books. For example:
Code:
epub:@EPUB_file:1:fbreader.app,AdobeViewer.app:ICON_EPUB
would make fbreader the default application for EPUB books.
If you search for extensions.cfg in this forum, I'm sure you'll see many past posts describing how this file can be used. It used to be a hot topic when 3rd party applications were more popular in the past.
As far as overriding the CSS in an EPUB goes, I'm not an expert in that area. I was just saying what I had read in past posts here. I don't know why some applications don't allow the CSS in a book to be overridden, but I'm sure there was some reason for it.