Unfortunately I don't know how successful you will be with the books you have converted using Calibre. I played around quite alot to get a fb2.css file working well with Calibre converted fb2 but unfortunately Calibre does not build the fb2 file to proper standards (see this thread
https://www.mobileread.com/forums/sho...d.php?p=903993).
From my findings and help from others, Book Designer seems to be the best application out there to create fb2 files. Once you have a fb2 file built to standards, creating a fb2.css (or using an existing one) is an absolute easy process.
Hope it is of some help, and hopefully others might be able to shed some more light or offer some alternatives.