IMO a monitor should be setup to truthfully show all colors and with right gamma. Screwing up the settings just because its fashionable to use white background on most websites feels very wrong.
Though its too widespread nowadays, so just changing one website will not help very much, therefore turning on the light to not sit in a completely dark room is a good idea.
I once read it was possible to supply a local css file for a browser to replace all websites css files for people that cant see well, maybe thats an option for you.