Dark GTK Themes do not work in Flatpak

[ilikelinux]

Spotify and Visual Studio Code no longer have dark title bars

This is looking more like an issue with GNOME shell. I filed bugs against Flathub’s Spotify and the GTK Dark titlebar VSCode Extension and both found the issue appears to be caused by upstream GNOME changes dropping support for GTK3. I suspected as much; the titlebars of all Electron-based applications appear to be GTK4 windows now; not GTK3. I don’t have a solution to this. So for now, we’ll have to live with mismatchy titlebars.