Here is the `Custom kepub default margins` patch for firmware 3.2.0. It should set the base left/right margin for kepubs, i.e. the margin that remains when all margins have been removed from the book and the adjustment slider is at the far left. It won't affect the top/bottom margins. (In firmware 3.3.0 and later it affects the top/bottom margins in full-screen reading mode though.)
I haven't tested this patch with this firmware version, so let me know if there are problems.
firmware 3.2.0: