I stumbed upon a rather interesting built-in feature on my Linux box.

All of the colors—images, text areas, icons, &c—have been inverted.  Perhaps you’d like to see your Opera perform like this?  Or any other application on your computer?

This would be great for folks with visual difficulties relating to color perceptions (like color-blindness).  I’m not clear whether this is something particular to Ubuntu, Gnome, or Compiz, but it is easy to toggle on and off.

Merely hit super-n (Windows-n) on the application in question.  In the example above I have done this in Opera, but I tested it in Thunderbird, AbiWord, Terminator, and (oddly enough) a remote session (where it effected all the visuals for the remote system through the RDP connection).

Mac has some similar functionality.  If you use ctrl-opt-apple-8 the OS will invert the colors too.  The difference is the Mac version applies across the entire desktop (background, doc, &c), but the Ubuntu version is application specific.

Happy hunting.