How To Get Flatpak Apps To Use The Correct Gtk Theme
https://www.linuxuprising.com/2018/05/how-to-get-flatpak-apps-to-use-correct.html
‘Flatpak is great for many reasons, but there's one annoyance. Due to sandboxing, Flatpak applications use the default Gnome theme (Adwaita), and not whatever Gtk theme you're currently using.
The fix for this issue is to install your current Gtk theme from Flathub. This way, Flatpak applications will automatically pick up your current system Gtk theme and use that instead of Adwaita.’