where can I change the theme used by pavucontrol?

I've already tried xfce4-appearance-settings, systemsettings and gnome-tweak-tool

and the later shows the same problem.
In .config/gtk-3.0/settings.ini I changed gtk-application-prefer-dark-theme=1 to gtk-application-prefer-dark-theme=0, but that did not help.
(running cauldron)
TIA.