[ibu]

There's only one way to control ugly css styles of the producer of epubs with a global user css style:
you have to convert the *.epubs into *.kepub.epub

That done with the help of the Calibre plugin "KoboExtended".
For Kepubs you have the option to use
\.kobo\kepup\kepub-book.css

Bad luck:
The plugin adds an header. And it changes the footer to the numbering "page/chapter".

I need the css file but I don't want a footer nor a header.

I like a true fullscreen mode. Nothing but content.

Any ideas how to realize that?
Thanks.