It would be nice if you could customize the color of the page background and text but I understand that you are limited by Firefox itself when it comes to that now.
Edit:After looking at my older extensions on a previous version of FF I realize this was done with the addon Better Reader
I have been a long time user of this addon and am glad you have kept working on it.

Does exactly what it says it will do and I would recommend it to anybody else that likes to use the Reader view.