And it worked. So, as far as I understand, I can't just copy/paste code, you have given to me. Perhaps, there is some kind of protection of this property. What should I do then?
It is so strange that nobody asked for this before. Maybe, they give up as early, as see some difficults but I really need this. My game works in 1920x1080 and on some bigger screens it leaves a lot of black space outside viewport(I understand that I can change color of this space in project properties but it will be forced for all layouts. In my situation I have few layouts with different background color, so if I choose one - it won't fit to other layouts). Also, I can't just use "Scale outer" fullscreen mode, because it brakes many game layouts and nearly half of the game should be made just from beggining. It is very bad idea - I have already spent nearly 2000 hours on development and now I am making final changes(colors of background is one of them).
I am begging you: help me to resolve my puzzle. I have read half of Google and found nothing. I don't know that to do now.