You probably don’t physically reach for GURPS Darkness because it’s just a 17 page PDF, but those 17 pages are jam-packed with content including 6 distinct varieties of darkness.

Edit: I forgot that most GMs don’t even use GURPS Darkness. If you go to the Steve Jackson Games forums you’ll find a 15 year old discussion building darkness from scratch, ending with Kromm coming in to say he just runs it as a Ghost with Weakness(Light) and gets on with his day.