I dislike that most websites use a black font on white background for their sites, as it gets too tiresome for me to read.

Back in the days of 11.04, using Gnome2 with compiz, there actually was a Negative feature that could negate the content of any window, making the background black and the font white. Much easier on the eyes for me.

Yet since 11.10, using gnome shell with mutter, I have no idea if there is something alike out there.

Hence my question: How do I negate the currently active window in gnome shell?

I am not interested in alternative methods, e.g. user styles. I am aware of their existence but I find it much easier to just invert the screen by the hit of a key shortcut. I also want the solution to be application-agnostic. As I also from time to time would want to invert libre-office or some other glaringly white application.