Option-Command-Control-8

If you have a Mac and have never tried that key combination, do it once just for fun.  It inverts the colors on your screen.

Interestingly, when I try to take a screencap of the result, the result is a non-inverted positive image, so screencaps for some reason don't capture the effect.

Does this work on a PC?  Or is there a different key combination in those systems?

Repeat the key combination to reverse the effect, btw.