another wrinkle....
In Budgie, I open terminal then launch gnome-control-center, and toggle to Appearance and choose Dark Theme.
Kill app. Open budgie-desktop-settings
. "Dark Theme" is always toggled On--I never play with that. While I'm in there I play with different themes (lovn the Yarus and Surfns).
I watch the windows and icons and widgets and open applications change with all my playing.
if you are waiting for the weird part, here it is: the only thing that does not change during theme change time is Nautilus. It's Dark, yes, but never changes. Hell I don't even know what dark theme it is in. It won't switch to a light theme either.
extra wrinkle: the folders icons will change theme when toggling themes but not the dark theme background.
Conclusion: gnome-control-settings
is the real boss of Nautilus? It is only obeying that app right now.