Firefox colors?

Does anyone know how to set web page colors in Firefox? That is, I want all web pages to display as white (or colored) text on a black background. The normal way of doing this (Edit->Preferences->Content->Fonts&Colors) has no effect at all since I upgraded my system to OpenSuSE 13.1. (Though it worked with earlier OS & Firefox versions.)

I'm running Firefox 28.0, the most recent version (updated today) available through Yast.