[electromagnetic]

I do exactly the same. I suppose using a darker background with black text has essentially the same effect as dimming the screen. One is limiting the light emitted from the backlight, the other is limiting the light transmitting through the liquid crystals. Both result in less light reaching your eye.

However, I suppose the dark background allows for immediate switching from low brightness to high brightness. If you're doing the 30/30 work cycle, you could easily switch from a dark coding screen to a proper brightness game or tv show.