feat(qt): integrate qt hm module with NixOS for sensible defaults

These changes make the PR apply the Kvatum theme only when appropriate
and creates the framework to allow for Qt styling based on the DE.

modules/qt/hm.nix

@@ -3,62 +3,88 @@
   config,
   lib,
   ...
-}: {
+}:
+{
   options.stylix.targets.qt = {
     enable = config.lib.stylix.mkEnableTarget "QT" pkgs.stdenv.hostPlatform.isLinux;
-    iconThemeName = lib.mkOption {
-      description = "Default QT Icons";
+    platform = lib.mkOption {
+      description = ''
+        Platform for QT.
+
+        Defaults to the standard platform of the configured DE in NixOS when
+        `stylix.homeManagerIntegration.followSystem = true`.
+
+        Fallback to qtct.
+      '';
       type = lib.types.str;
-      default = "ePapirus-Dark";
+      default = "qtct";
     };
   };
 
-  config = lib.mkIf config.stylix.targets.qt.enable (let
-    cfg = config.stylix.targets.qt;
-    kvconfig = config.lib.stylix.colors {
-      template = ./kvconfig.mustache;
-      extension = ".kvconfig";
-    };
-    svg = config.lib.stylix.colors {
-      template = ./kvantum-svg.mustache;
-      extension = "svg";
-    };
-    kvantumPackage = pkgs.runCommandLocal "base16-kvantum" {} ''
-      directory="$out/share/Kvantum/Base16Kvantum"
-      mkdir --parents "$directory"
-      cat ${kvconfig} >>"$directory/Base16Kvantum.kvconfig"
-      cat ${svg} >>"$directory/Base16Kvantum.svg"
-    '';
-  in {
-    home.packages = with pkgs; [
-      qt5ct
-      libsForQt5.qtstyleplugin-kvantum
-      qt6Packages.qtstyleplugin-kvantum
-      kvantumPackage
-      papirus-icon-theme
-    ];
+  config = lib.mkIf config.stylix.targets.qt.enable (
+    let
+      cfg = config.stylix;
+      iconTheme =
+        if (cfg.polarity == "dark") then cfg.iconTheme.dark else cfg.iconTheme.light;
 
-    qt = {
-      enable = true;
-      platformTheme.name = "qtct";
-    };
+      kvantumPackage =
+        let
+          kvconfig = config.lib.stylix.colors {
+            template = ./kvconfig.mustache;
+            extension = ".kvconfig";
+          };
+          svg = config.lib.stylix.colors {
+            template = ./kvantum-svg.mustache;
+            extension = "svg";
+          };
+        in
+        pkgs.runCommandLocal "base16-kvantum" { } ''
+          directory="$out/share/Kvantum/Base16Kvantum"
+          mkdir --parents "$directory"
+          ln -s ${kvconfig} "$directory/Base16Kvantum.kvconfig"
+          ln -s ${svg} "$directory/Base16Kvantum.svg"
+        '';
+    in
+    {
+      warnings = lib.optional (cfg.platform != "qtct") ''
+        Stylix has not yet implemented qt styling for any platforms other than "qtct".
+        We are working on it.
+      '';
 
-    xdg.configFile."Kvantum/kvantum.kvconfig".source = (pkgs.formats.ini {}).generate "kvantum.kvconfig" {
-      General.theme = "Base16Kvantum";
-    };
+      home.packages = lib.optional (config.qt.style.name == "kvantum") kvantumPackage;
+
+      qt = {
+        enable = true;
+        style.name =
+          if cfg.targets.qt.platform == "qtct" then "kvantum" else lib.mkDefault null;
+        platformTheme.name = cfg.targets.qt.platform;
+      };
 
-    xdg.configFile."Kvantum/Base16Kvantum".source = "${kvantumPackage}/share/Kvantum/Base16Kvantum";
+      xdg.configFile = lib.mkMerge [
+        (lib.mkIf (config.qt.style.name == "kvantum") {
+          "Kvantum/kvantum.kvconfig".source =
+            (pkgs.formats.ini { }).generate "kvantum.kvconfig"
+              {
+                General.theme = "Base16Kvantum";
+              };
 
-    xdg.configFile."qt5ct/qt5ct.conf".text = ''
-      [Appearance]
-      style=kvantum
-      icon_theme=${cfg.iconThemeName}
-    '';
+          "Kvantum/Base16Kvantum".source =
+            "${kvantumPackage}/share/Kvantum/Base16Kvantum";
+        })
 
-    xdg.configFile."qt6ct/qt6ct.conf".text = ''
-      [Appearance]
-      style=kvantum
-      icon_theme=${cfg.iconThemeName}
-    '';
-  });
+        (lib.mkIf (config.qt.platformTheme == "qtct") {
+          "qt5ct/qt5ct.conf".text = ''
+            [Appearance]
+            style=${config.qt.style.name}
+            icon_theme=${iconTheme}
+          '';
+          "qt6ct/qt6ct.conf".text = ''
+            [Appearance]
+            style=${config.qt.style.name}
+            icon_theme=${iconTheme}
+          '';
+        })
+      ];
+    }
+  );
 }

modules/qt/nixos.nix

@@ -0,0 +1,57 @@
+{
+  lib,
+  pkgs,
+  config,
+  ...
+}:
+
+let
+
+  recommendedStyle = {
+    gnome = if config.stylix.polarity == "dark" then "adwaita-dark" else "adawaita";
+    kde = "breeze";
+    qtct = "kvantum";
+  };
+
+in
+{
+  options.stylix.targets.qt = {
+    enable = config.lib.stylix.mkEnableTarget "QT" pkgs.stdenv.hostPlatform.isLinux;
+    platform = lib.mkOption {
+      description = ''
+        Platform for QT.
+
+        Defaults to the standard platform of the configured DE.
+
+        Fallback to qtct.
+      '';
+      type = lib.types.str;
+      default = "qtct";
+    };
+  };
+
+  config = lib.mkIf (config.stylix.enable && config.stylix.targets.qt.enable) {
+
+    stylix.targets.qt.platform =
+      if config.services.xserver.desktopManager.gnome.enable then
+        lib.mkDefault "gnome"
+      else if config.services.xserver.desktopManager.plasma5.enable then
+        lib.mkDefault "kde"
+      else if config.services.xserver.desktopManager.lxqt.enable then
+        lib.mkDefault "lxqt"
+      else
+        lib.mkDefault "qt5ct";
+    qt = {
+      enable = true;
+      style = lib.mkIf (
+        recommendedStyle ? "${config.qt.platformTheme}"
+      ) recommendedStyle."${config.qt.platformTheme}";
+      platformTheme =
+        if config.stylix.targets.qt.platform == "qtct" then
+          "qt5ct"
+        else
+          config.stylix.targets.qt.platform;
+    };
+
+  };
+}

stylix/home-manager-integration.nix

@@ -174,6 +174,14 @@ let
             "polarity"
           ];
         }
+        {
+          path = [
+            "stylix"
+            "targets"
+            "qt"
+            "platform"
+          ];
+        }
       ];
 
 in