, black means no light is being sent into your eyes,
Which also means: your pupils having to open wider (as they do if it is darker). Which in turn means the focal field (not sure if that is the correct English term) becomes more narrow, and your eyes have to put in more work to keep the exact focus.
As in a camera a narrow small aperture, so does a "small pupils" in the eye increase the area (min to max distance) within which something is focused. With a bigger aperture things have to be exactly on the right distance, and all else starts bluing... => more light, wider pupils, less focal adjustment by the muscles controlling the lens in your eye.
A white background should not be tiring, if
- its overall brightness is moderate (many monitors come with defaults that act as floodlight..., but that can be adjusted).
- the room surrounding you is equally well lit (so your pupils are as small as possible / helping the focusing)
- the lighting is set up to avoid glaring and reflection
Of course, if you switch, your brain needs to adjust to interpret the reverse image. That is tiring for some time too, though not to the eyes.