Sorry to invade the conversation, but do you know Focus Writer? It's an open source program that I like, but I don't use it because it doesn't have auto-save. I've already lost 1K words when the power went out.
It has the “Timer” in the corner of the screen and when activated, it turns black and loses color like clockwork. When I read @still_life suggestion I immediately thought about this program.
Exemple: