Mupdf css style sheet support
Mupdf allows you to change the general appearance of epubs through a custom css style sheet (see e.g. relevant c file, and page 43 of this article on mupdf). An other mupdf-based document reader that does this is plato, which uses a file called
user.css for this purpose.
I'm not super keen on the default look of epubs in Zathura. I have no idea how to change the font, and how to get rid of ugly header colors I sometimes see. It therefore might be beneficial to include support to change this via an zathura setting that points to a default