Since Xubuntu 19.10 beta the xfce4-session-logout program has a light gui theme. Previously it was dark with the Numix theme. When I set gtk-application-prefer-dark-theme from GtkSettings with the Gtk Inspector to true then everything looks fine.
Also the whisker menu is completely light and need un update.
I think the widgets would fit in much better with more dark styling or dark themes.
You are using an old version. Wisker menu support already landed in #666. Can you use the PPA ? Session-log-out dialog needs a root class to override the style. I can't find that atm.
Since Xubuntu 19.10 beta the xfce4-session-logout program has a light gui theme. Previously it was dark with the Numix theme. When I set gtk-application-prefer-dark-theme from GtkSettings with the Gtk Inspector to true then everything looks fine.
Also the whisker menu is completely light and need un update.
I think the widgets would fit in much better with more dark styling or dark themes.