JoeyBlaze..
I'll suggest that you try to use a San-Serif font (they are easier for the eye)..
In my Fire, I use Arial, in the smallest size, with the smallest line-space in WHITE over DARK background..
I've been reading ebooks since my very very old Palm, and then Clié and then iPod Touch and then Kindle keyboard and now Fire (and I'm still using my iPod)...and yes..I still have good eyes..
In my humble experience, the best combination (thinking about stability for your eyes) is to have white letters over dark background with a "simple" and plain font-type..
I hope this can be useful