Paper can be very white and bright if well lit. Much more than a typical LED/LCD screen. If your screen is brighter than a white sheet of paper, it's probably too bright and you are probably tiring your eyes.
The only moment when a sheet of paper is darker than my screen is when I'm in my bed, in a completely dark room watching or reading something, and that's not known as a very good thing to do for the eyes (and for the body posture neither by the way).
I'm not buying the fact we should darken the background of a text. I think we should let people reduce the brightness of their screen instead.