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. xfce4-session-logout has a bright gui even when I select a dark theme like "Adwaita dark".
Created attachment 9093 dark gui with gtk inspector
Created attachment 9094 current light gui
Now that xfce4-session is built against GTK3 I assume that themes like Numix need to be updated. The following style changes were added for GTK2: https://github.com/numixproject/numix-gtk-theme/commit/c828372ea652098da08a7867f6dd68e7294e3e3b Also, I checked with Adwaita-dark and it does change the appearance accordingly.
I created this bug over at the Numix project: https://github.com/numixproject/numix-gtk-theme/issues/739 So you get a dark xfce4-session-logout program when Numix is selected as the theme?
Created attachment 9101 Adwaita-dark No issue with Adwaita-dark here. Tested with Xubuntu 19.10 inside a VM.
How is the gui theme if you select the Numix theme?
Same as shown in your screenshot (comment #2). I did not say that Numix gives me a dark appearance for the GTK3 logout dialog, but Adwaita-dark does.
Okay, feel free to change the status of this bug report.