I would explain it in terms of paper. I have yet to meet someone who illuminates paper by shining a bright white light from behind it. Instead it is lit by dayl

I would explain it in terms of paper. I have yet to meet someone who illuminates paper by shining a bright white light from behind it. Instead it is lit by daylight or reading lights, which are often a shade that makes the paper seem pleasantly off-white. It follows that making your computer or web design a slight off-white is easier on the eyes, particularly in the case of a text-heavy design.