Fixed toolbar's disabled button color.

Before, it would simply desaturate the buttons when they were disabled (which was an issue with the black-and-white buttons). Now it simply lightens it like it should. 

A workaround had to be used that explicitly sets the buttons' disabled bitmaps to a lightened version. This happens automagically, though, so there's no need to bundle disabled icons with the themes. 

This has been tested on Linux, Windows, and OS X. 
Is it possible to pass original by const reference here?