In normal it is light-grey. Mostly white. But, when I working for a long time, I feeling, that my eyes are overstressed. So now? I find the salvation of the problem like this (File: Dest.png). But, I need from you - ability to optional changing color of the background.(For example somebody need another color, or need to print the lesson) - So that's why.
Also, I wait from you making grid more contrast