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I installed elementary OS 6.0 Odin (final release 2021-08-10).
I was finally able to get FlatPak apps to install. However, none use the elementary OS Dark Theme. I checked FlatPak and the elementary theme is installed. How do you get Odin to use the Dark Theme system wide for all FlatPak apps?
I also needed to install a dark theme to flatpak with
flatpak install flathub org.gtk.Gtk3theme.Adwaita-dark
Then using
flatpak list
to get the application ID... Then the following command to apply the theme to the application ID... For example with LibreOffice
flatpak override --user --env=GTK_THEME=Adwaita-dark org.libreoffice.LibreOffice
That command worked great in Hera, but it seems to be hit or miss in Odin. It will hold for a while... even occasionally after a reboot, but ultimately it seems to lose the setting and default back to the flatpaks "light" setting.
Adding GTK_THEME=Adwaita-dark as an Environment Variable in Flatseal seems to yeild the same results. Works for a while but in Odin, it seems to disappear. In Hera it appears permanent.
You can force to use dark stylesheet using Pantheon Tweaks which will create new local settings under ~/.config/gtk-3.0/settings.ini with content below.
[Settings]
gtk-application-prefer-dark-theme=1
Here's another approach:
"Uniform theme and fonts for Qt apps in GTK environments" provided by the EndeavourOS-guys: Instead of the the given arch-linux install cmd ('pacman -S pkg-name') you have to replace it with your normal install cmd ('apt install pkg-name'), and then follow the given instructions for user-space-setup (quickest).
flatpak, while your answer uses apt. Moreover, the issue is with all apps, including GTK apps like Pitivi. This is flatpak-specific issue, really.