I do not believe NixOS or Guix are shipping these packages with a sandbox that uses the portals, but I could be wrong. I am not sure of anything about Snap, I h

I do not believe NixOS or Guix are shipping these packages with a sandbox that uses the portals, but I could be wrong. I am not sure of anything about Snap, I have never used it. In Flatpak, all the config files are saved to ~/.var/app/. The portal should be able to handle exposing a file from the host, but the app has to be built to use the portal correctly. You can have themes but you must install the flatpak package for them, pulling themes from the host environment is a security issue because technically themes are equivalent to doing code injection.