Hey Montyp535
I believe you put your fonts in a "fonts" folder on the root of the SD Card turn the device off and on.... don't quote me on this as i have not tried it ! but that's what it says in the Manual.
I can read any book in epub format and have no issues with css.. i do however re format every book using kindle setting but in epub format using the Calibre program ? maybe you could try that if your having Issues with the way the book looks on the device?
At this stage I do not believe you can have the book covers as thumbs.. that would be grand if i could !
Just for further info they brought a new firmware out today V1.8 @
ftp://125.91.8.7/
User Name - FTP_user1
Password - user123