Can't remember which books in my library had issues with fonts, which with margins, etc. Any downside to converting all the books in this fashion (checking all the boxes)? i.e. can there be any problems if I take a book that's completely fine, check all the boxes, and convert it?
I can only speak for myself, but I had various issues with Kobo and other e-readers when dealing with side loaded books, fonts that could not be changed, margins, line-spacing etc.
Since I updated the look and feel as taming posted - I no longer have any issues modifying fonts, margins, etc. The only remaining issue is the bottom margin and that was a problem before I made the changes.