I find it much easier to write in pale yellow or white text on a black background. The white is too bright especially if I've been working all day and my eyes are already tired from staring into a computer screen. Turning the brightness down doesn't help because I already have to strain to see, the dim screen makes it that much harder. When I write in the mornings or at my second favorite coffee shop I'm in dim light and that makes the screen hard to see then as well.
Being able to reverse the writing background eases the strain and for what ever reason is just easier for me to think.