I wasn’t able to get the gsettings method to work (I’m on Wayland KDE), and that article doesn’t say anything about theming QT Flatpaks. Also, after “installing” my GTK theme as a flatpak via the method described, it still wasn’t available to my GTK Flatpaks via the GTK_THEME method. The steps in the itsfoss.com article do work, though there’s been a lot of squabbles about the “proper” way to expose themes to Flatpaks. Regardless, this all goes back to my point that theming Flatpak is clunky and should be much smoother.