Here is a patch that does the same as the one above for KePubs in full-screen mode. Unfortunately KePubs in normal mode use a different method of setting the margins, so it looks like a more complicated patch will be needed to achieve the same thing in normal mode.
Note that in full-screen mode the
`Custom kepub default margins` patch affects the minimum size of all four margins, and the
`Fix three KePub fullScreenReading bugs` patch increases the size of the left/right margins a little.
(If you test this patch, remember that there is an unfixed bug in the KePub full-screen mode that prevents the margins being adjusted after loading a new chapter, so you might need to close and re-open the KePub book to be able to adjust the margins.)
`KePub full-screen mode margins step size`: (libnickel.so.1.0.0.patch)