How do I theme the Sound Menu playback buttons?

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http://askubuntu.com – In my GTK+2/3 theme I am making for Ubuntu 12.04 LTS, there's a very noticeable problem when it comes to the Sound Menu buttons - they are completely black with white icons: Obviously, this does not fit in at all. But where do these buttons get their theming from in my theme files? What section do I need to include so these can be themed? (HowTos)