Changing stuff for yourself is pretty easy. The function 
	Code:
	Font:getFace(font, size, faceindex)
  uses a font face & size maps: 
https://github.com/koreader/koreader...t.lua#L39-L104
You just need to replace the font faces & sizes on each Font:getFace() invokation you'll want to change.
Submitting changes upstream is harder, as you need to reach consensus.