[PSA] Making Flatpaks use system themes for QT and GTK is actually super easy!

^{Obligatory: Barely an inconvenience.}

You just need to tell Flatpak apps where to look for it:

1. Set a couple overrides to expose the GTK and QT configurations in ~/.config.
2. For GTK, find and install the GTK themes as Flatpaks from Flathub.
3. For QT, install every org.kde.KStyle.Kvantum Flatpak package. (Different apps may use different runtimes, so just grab all the versions.)
4. For QT, also copy the QT themes from /usr to ~/.config if they aren't already there.

This assumes that your system is already set up to use Kvantum in QT5CT/System Settings/Whatever— I recommend that anyway as it's worked pretty well for me.

So after doing this, the global system state should be something like below:

$ cat /var/lib/flatpak/overrides/global [Context] filesystems=xdg-config/gtk-4.0:ro;xdg-config/gtk-2.0:ro;/usr/share/Kvantum:ro;xdg-config/gtkrc-2.0:ro;xdg-config/gtkrc:ro;xdg-config/gtk-3.0:ro;xdg-config/qt5ct:ro;/usr/share/qt5ct:ro;xdg-config/Kvantum:ro; # I don't think the /usr overrides actually work, but let's leave them here as a reminder to copy from /usr/share/Kvantum to .config/share/Kvantum.

$ flatpak list | grep -i theme Breeze GTK theme org.gtk.Gtk3theme.Breeze 5.26.3 3.22 system Matcha-aliz GTK theme org.gtk.Gtk3theme.Matcha-aliz 2021-12-25 3.22 system Matcha-dark-aliz GTK theme org.gtk.Gtk3theme.Matcha-dark-aliz 2021-12-25 3.22 system Matcha-light-aliz GTK theme org.gtk.Gtk3theme.Matcha-light-aliz 2021-12-25 3.22 system Adwaita theme org.kde.KStyle.Adwaita 5.15-21.08 system Kvantum theme engine org.kde.KStyle.Kvantum 1.0.0 5.15 system Kvantum theme engine org.kde.KStyle.Kvantum 1.0.6 5.15-21.08 system Kvantum theme engine org.kde.KStyle.Kvantum 1.0.6 5.15-22.08 system

(Code formatting may be broken on the old Reddit frontend.)

This has been posted about before, but never in one place AFAIK, and there's a lot of conflicting information out there (primarily more temporary approaches that require manual setup for every Flatpak app on every theme change).

QT: