But gnome apps don’t do that on XFCE. simple-scan and zenity as an example.
The latest version of both simple-scan and zenity do support custom accent colors. AFAICT, XFCE doesn’t support the XDG accent color setting.
Please don’t implement libadwaita, guys.
This is just extremely misleading. Libadwaita uses the system accent color by default which makes it even easier respect the users preference when developing a GTK application.
The Flatpak is official.