nix-config/home/themes/dark/qt.nix

{ pkgs, ... }:
{

  home.packages = with pkgs.libsForQt5; [
    qt5ct
    qtstyleplugin-kvantum
  ];

  qt = {
    enable = true;
    platformTheme.name = "qtct";
    style.name = "kvantum";
  };

  xdg.configFile = {
    "Kvantum/ArcDark".source = "${pkgs.arc-kde-theme}/share/Kvantum/ArcDark";
    "Kvantum/kvantum.kvconfig".text = "[General]\ntheme=ArcDark";
  };
}