chromium: add plasmaSupport option for google-chrome

This flag allows Chrome to dynamically link to QT in order to introspect
your Plasma theme and make Chrome match.

Note, you may still need to add this entry to your home-manager config:
```
home.sessionVariables = [
  QT_QPA_PLATFORMTHEME = "kde";
];
```

modules/misc/news/2026/01/2026-01-19_05-36-20.nix

@@ -0,0 +1,19 @@
+{
+  time = "2026-01-19T05:36:20+00:00";
+  condition = true;
+  message = ''
+    Chrome now has a `plasmaSupport` flag.
+
+    If you use KDE Plasma, set:
+
+    ```
+    programs.google-chrome.plasmaSupport = true`
+    home.sessionVariables = [
+      QT_QPA_PLATFORMTHEME = "kde";
+    ];
+    ```
+
+    This enables the "Use QT" theme in **Settings > Appearance**, which makes
+    Chrome match your Plasma theme.
+  '';
+}

modules/programs/chromium.nix

@@ -16,6 +16,10 @@ let
     vivaldi = "Vivaldi Browser";
   };
 
+  plasmaSupportedBrowsers = [
+    "google-chrome"
+  ];
+
   browserModule =
     browser: name: visible:
     let
@@ -66,6 +70,15 @@ let
         '';
       };
     }
+    // lib.optionalAttrs (lib.elem browser plasmaSupportedBrowsers) {
+      plasmaSupport = mkOption {
+        inherit visible;
+        type = types.bool;
+        default = false;
+        example = true;
+        description = "Whether to enable the 'Use QT' theme for ${name}.";
+      };
+    }
     // lib.optionalAttrs (!isProprietaryChrome) {
       # Extensions do not work with Google Chrome
       # see https://github.com/nix-community/home-manager/issues/1383
@@ -152,6 +165,7 @@ let
           List of ${name} dictionaries to install.
         '';
       };
+
       nativeMessagingHosts = mkOption {
         type = types.listOf types.package;
         default = [ ];
@@ -230,10 +244,18 @@ let
       ];
 
       programs.${browser}.finalPackage =
-        if cfg.commandLineArgs != [ ] then
-          cfg.package.override {
-            commandLineArgs = lib.concatStringsSep " " cfg.commandLineArgs;
-          }
+        if cfg.package == null then
+          null
+        else if cfg.commandLineArgs != [ ] || (cfg.plasmaSupport or false) then
+          cfg.package.override (
+            lib.optionalAttrs (cfg.commandLineArgs != [ ]) {
+              commandLineArgs = lib.concatStringsSep " " cfg.commandLineArgs;
+            }
+            // lib.optionalAttrs (cfg.plasmaSupport or false) {
+              plasmaSupport = true;
+              kdePackages = pkgs.kdePackages;
+            }
+          )
         else
           cfg.package;