Prefer dark variant support for Flatpak sideloaded apps in Hera

Question

I'm running Elementary OS for some years now (since Freya) as my main OS and currently I'm on Hera (with the latest updates). Recently I decided to switch some applications that I was installing from custom or 3rd party PPAs to their Flatpak counterpart since they are more up to date and in many cases they are official releases from the developer teams which I trust more.

I'm using the "Tweaks" add-on and I have set there the "Prefer dark variant" setting to true. It works great so far as any application that supports a dark theme or mode will use that by default (ie: Firefox, Files, Mail, Remmina, etc).

Now the issue is that for Flatpak applications it doesn't work. For example Remmina will use the light theme. I tried the same apps installed through Snap instead of Flatpak and they do honor the "Prefer dark variant" setting.

I checked within Flatpak (flatpak list) and I found that elementary stylesheet is installed:

Name                                             Application ID                                Version            Branch          Installation
elementary stylesheet                            org.gtk.Gtk3theme.elementary                                     3.22            system

I also try to modify ~/.config/gtk-3.0/settings.ini by adding:

[Settings]
gtk-application-prefer-dark-theme=1

But it didn't help either.

Now my questions are:

• Is this a known limitation of flatpak apps in Hera?
• Is this caused by "dark theme" information missing from elementary stylesheet in the flatpak package?
• I'm missing something? Some additional configuration?

Thanks

Answer 1

The following worked for Xournal++, so there is a chance it will work for other apps too:

flatpak install org.gtk.Gtk3theme.Adwaita-dark
sudo flatpak override --env=GTK_THEME=Adwaita-dark com.github.xournalpp.xournalpp

You can change com.github.xournalpp.xournalpp to the app you are trying to change to dark mode. You can also change Adwaita-dark to any theme you like – you can see the list of available themes using the following command:

flatpak remote-ls flathub | grep org.gtk.Gtk3theme