This one is courtesy of my friend Andrea, whose cat manages to press the wrong keys when it steps on the keyboard.
There’s an option to invert the screen on a Mac (as shown above). You can invert the screen by going to the Apple menu, then System Preferences, then Accessibility (unless you have an older system, in which case it’s called “Universal Access”). Once you get there, click on Display, and check the box for “Invert Colors” to get a very interesting display. You probably will want to uncheck that box shortly.
Andrea’s cat did it the easy way: Control-Option-Command-8. That keyboard shortcut toggles between normal view and inverted view. It might not work for you (which would be good, if you have a cat). If it doesn’t work look in System Preferences/Keyboard/Keyboard Shortcuts, then click on Accessibility, then check the box for Invert Colors. NOW you can do Control-Option-Command-8 to toggle between normal and inverted. Whee!