It does what it says but it doesn't work on addons.mozilla.org. This makes me suspect it won't work in many other pages.

Also I don't quite understand hwo to use web-ext. Is the product not targeted for me or..? It

Works great on most pages .. But you have to remember to disable this if you like watching youtube videos in fullscreen mode (else you just get a blank screen).

Does what it says, it inverts images witch I wish it didn't, it will also be disable when typing.

It's a good idea, but it makes all the images look like negatives. It needs separate toggles for images and backgrounds. Until then, I'm better off using the other add-ons.