My apps installed with flatpak don't use the correct GTK theme. As stated here: https://github.com/flatpak/flatpak/issues/1147 the theme should also get set over XSettings.
Yes, I have no idea how that works. If someone can provide a patch or at least send us a snippet, that would be very much appreciated.
As a workaround, I got it working with the following steps: 1. Install xsettingsd (https://github.com/derat/xsettingsd , but is also part of many standard repos) 2. `echo 'Net/ThemeName "Arc-Dark"' >> ~/.xsettingsd` (Change Ark-Dark to whatever theme you have installed) 3. Stard xsettingsd (just run the command `xsettingsd` as current user) 4. It should immediately work for every flatpak application which is started aferwards
FWIW, this was fixed for Wayland with https://cgit.kde.org/kde-gtk-config.git/commit/?id=87040960da7e9e587b4cb61251807e164aa53483, so the issue now only persists for X11. Re-titiling the bug appropriately.
You can also make this work by running $ GTK_THEME=Breeze flatpak run ...
*** This bug has been marked as a duplicate of bug 375272 ***