One of the reasons I switched to Fedora recently was it’s support for Flatpaks. Sure they’ll work anywhere, but having them be such a big part of Fedora (check out Fedora Kinoite) really intrigued me. I love the idea of having the same version of apps across distros that’s also the latest (or nearly) version of the app.

I also like when every app looks like it’s part of the same ecosystem. The KDE and Gnome devs have done an awesome job of that by making themes that work in both GTK and Qt (KDE) apps. Unfortunately, Flatpak apps using GTK on KDE don’t automatically use the theme you’ve set in System Settings.

To fix that, I’ll show you how you can use the Flatseal app to set the GTK them for Flatpak apps. You can do this through the shell for individual apps and universally, but I prefer this way to give myself flexibility.

I’m assuming you’re using Breeze or Breeze-Dark here. Otherwise, substitute with the theme you are using. If the theme you are using is not in flathub, you might need to choose one that is.

sudo flatpak install org.gtk.Gtk3theme.Breeze
sudo flatpak install org.gtk.Gtk3theme.Breeze-Dark

Install the Flatseal app from flahub as well:

sudo flatpak install com.github.tchx84.Flatseal

Open the Flatseal app and choose Thunderbird on the left.
Scroll down to the variables section, slick the + symbol, and add:

GTK_THEME=Breeze-Dark

The next time you start Firefox it should be using the Breeze Dark Theme! You can do this for any flaptak app that uses GTK.