Anyone with serious visual impairments almost certainly already has a user stylesheet installed in their browser of choice.

I am a person with reasonably decent vision who is fatigued by high-contrast color schemes. I greatly appreciate grey-on-black and other such color schemes that make it so I'm not staring into a lightbulb for ten-to-twelve hours a day. (Yes, my monitor brightness is set to a reasonable level. Yes, black on white is still far more fatiguing than white on black.)