After the move to Plasma 6, a number flatpaks have not been displaying properly in dark mode. I searched in here and found this comment and was able to fix OnlyOffice. After that I noticed that all of the improperly acting apps are GTK apps, and I was able to "fix" them by applying the same flatpak override to each GTK based flatpak. Are others finding this needs to be done. Is there a way to get GTK flatpaks to be properly dark under Plasma without having to manually overwrite the theme for each one?