Flatpak not following GTK theme

I’ve recently installed NixOS on a brand new PC. I have been having quite a few struggles with theming my desktop. I at first struggled a lot trying to get basic apps to theme, but I fixed that by using themechanger. This created ~/.config/gtk-3.0, ~/.config/gtk-4.0 configuration files and set them properly. This works fine for regular installed apps, however for flatpaks it seems to not follow my theme at all.

I am running Hyprland with the following portals:

  • xdg-document-portal
  • xdg-destkop-portal-gtk
  • xdg-destkop-portal-wlr

Using Door Knocker gives me two warnings about not finding xdg-desktop-portal and it not being able to determine desktop portal version.

Part of configuration.nix file:

  # Portals
  xdg = {
    autostart.enable = true;
    mime.enable = true;
    menus.enable = true;
    icons.enable = true;
    sounds.enable = true;
    portal = {
      enable = true;
      wlr.enable = true;
      extraPortals = [

Is there a way I can tell Flatpak to use my system themes? Or fix my portals not functioning like normal (apparently).

Hi. This is impossible to do without different overrides and other hacky things now. BTW, what is a themechanger?