JToggleButton doesn't honor contentAreaFilled boolean on both Nimbus and GTK. Looking in SynthToggleButtonUI, the paintBackground method should check to see whether contentAreaFilled is true before painting.
Description is accurate. Simple fix, simply add a check for contentAreaFilled to paintBackground of SynthToggleButtonUI. SynthButtonUI has this check, SynthToggleButtonUI overrides this method from SynthButtonUI but fails to do the check.