qtstyleplugin-kvantum: fix loading builtin themes on Qt 6

Signed-off-by: Sefa Eyeoglu <contact@scrumplex.net>

pkgs/development/libraries/qtstyleplugin-kvantum/default.nix

@@ -13,6 +13,8 @@
 , qttools
 , wrapQtAppsHook
 , gitUpdater
+
+, qt5Kvantum ? null
 }:
 let
   isQt6 = lib.versionAtLeast qtbase.version "6";
@@ -59,6 +61,12 @@ stdenv.mkDerivation rec {
       --replace "\$\$[QT_INSTALL_PLUGINS]" "$out/$qtPluginPrefix"
   '';
 
+  postInstall = lib.optionalString isQt6 ''
+    # make default Kvantum themes available for Qt 6 apps
+    mkdir -p "$out/share"
+    ln -s "${qt5Kvantum}/share/Kvantum" "$out/share/Kvantum"
+  '';
+
   passthru.updateScript = gitUpdater {
     rev-prefix = "V";
   };

pkgs/top-level/qt6-packages.nix

@@ -41,7 +41,9 @@ in
 
   qtpbfimageplugin = callPackage ../development/libraries/qtpbfimageplugin { };
 
-  qtstyleplugin-kvantum = callPackage ../development/libraries/qtstyleplugin-kvantum { };
+  qtstyleplugin-kvantum = callPackage ../development/libraries/qtstyleplugin-kvantum {
+    qt5Kvantum = pkgs.libsForQt5.qtstyleplugin-kvantum;
+  };
 
   quazip = callPackage ../development/libraries/quazip { };