It's probably at least partly a matter of how the rest of your room is lit. Your sight is sharpest when your pupils are small, so a bright monitor in a bright room will work best for many people.
However, if you're looking at a dark monitor in a bright room, or at a bright monitor in a dark room, your pupils may be too large for the amount of light that reaches them to be entirely comfortable, which causes squinting and eyestrain.
A dark monitor in a dark room won't have that problem, but you may still be dealing with some increased blurriness because of your enlarged pupils, which may bother you depending on how good your eyesight is.
However, if you're looking at a dark monitor in a bright room, or at a bright monitor in a dark room, your pupils may be too large for the amount of light that reaches them to be entirely comfortable, which causes squinting and eyestrain.
A dark monitor in a dark room won't have that problem, but you may still be dealing with some increased blurriness because of your enlarged pupils, which may bother you depending on how good your eyesight is.