I just finished installing Cinnamon, added Papirus icon theme and also installed mint-themes and mint-y-icons from AUR, so all looks fabulous. As next step I installed deja-dup, but it absolutely ignores my theme settings - not only the Mint-Y window borders, but dark theme in general:
I understood that new Gnome applications are using libadwaita and can ignore system theme, but does anybody know if there is a workaround to force Cinnamon theme to Deja Dup?
Just wanted to share that I have at least a workaround. Apparently you can create your own /.config/gtk-4.0/gtk.css where you can define your own colors, which will be accepted by all GTK4 apps. For Mint-Y-Dark themes, this looks quite ok:
In the meantime, more and more Gnome apps in the repo are switching to libadwaita (e.g. gnome-calculator, baobab, deja-dup etc.) and all seem to be accepting the above settings.
I guess it is now up to the community to create gtk.css files compatible with various Cinnamon themes.
I hope this helps someone!