I'm using Papirus-Light, but changing the icon setting to Papirus-Dark or just Papirus doesn't change them. The GTK theme I'm using is Qogir-Win-Light, but changing that also doesn't change the tray icons, nor does the 'Dark theme' switch in budgie settings.
I can possibly try to just manually override them in .config/gtk-3.0/gtk.css, but I don't know what the name of the CSS style for those icons is to change it.