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. 
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