Reduce margins and normalize line spacing

Hi guys! Is it somehow possible to reduce the margins more than what's available in the kindle settings? I've got some epub books which I convert to kfx (used to convert to azw) with Calibre. I tried a few different methods but to no avail...

Also I need to switch the line spacing everytime I change a book as the option is systemwide... If there is a way to set the line spacing when I convert the books it would be awesome!