Kdialog theming not working through desktop portal

I’ve been trying to get kdialog/qt file picker working on the Vivaldi web browser, but I can’t get the theming to work: both the color scheme and icon set won’t load in. It looks fine on native qt apps though.

Here’s my configuration.nix

  # Environment variables
  environment.sessionVariables = rec {

# Desktop integration portals
  xdg.portal.extraPortals = [ pkgs.libsForQt5.xdg-desktop-portal-kde ];
  xdg.portal.enable = true;

Thank you for your time!

I’ve gotten it to work by setting qt5ct’s style to kvantum (pkgs.libsForQt5.qtstyleplugin-kvantum)
and theming from there.

Now I’m trying to get image previews to work…

Edit (14 hours later): I rebooted my computer and now it won’t work. I don’t understand.