Is your feature request related to a problem? Please describe.
I have found the PDF color inversion to work perfectly for example PDFs which consist of text with images throughout. However, for PDFs which are full page images, the color inversion does not activate and leaves the document rendered as it originally appears. To the best of my knowledge, the color inversion keybinding i has no effect on documents that fit this criteria (I seem to presently be suffering from #237 but I have been able to successfully toggle color inversion on other documents which consist of text with some images, so I think I would have been able to do the same with the documents that are full page images). I have gone through the list of keybindings and see no alternative color inversion which would force documents like this to be inverted.

Describe the solution you'd like
I would like to have a key binding which forcefully applies the color inversion irrespective of regions classified as images.

Describe alternatives you've considered
Another possible acceptable solution is to add a third toggle when using i which cycles through: dark (leave images alone), dark (including images), original color.