chore: add SetGTKDarkThemeEnabled(enabled) internal helper to allow dynamic theme selection on linux (#19964)

This is just a after-creation setter for the `darkTheme` constructor option.  This is delibrately
a method and not a property as there is no getter.

shell/browser/api/atom_api_top_level_window.cc

@@ -889,6 +889,10 @@ void TopLevelWindow::CloseFilePreview() {
   window_->CloseFilePreview();
 }
 
+void TopLevelWindow::SetGTKDarkThemeEnabled(bool use_dark_theme) {
+  window_->SetGTKDarkThemeEnabled(use_dark_theme);
+}
+
 v8::Local<v8::Value> TopLevelWindow::GetContentView() const {
   if (content_view_.IsEmpty())
     return v8::Null(isolate());

shell/browser/api/atom_api_top_level_window.h

@@ -202,6 +202,7 @@ class TopLevelWindow : public mate::TrackableObject<TopLevelWindow>,
   void SetAspectRatio(double aspect_ratio, mate::Arguments* args);
   void PreviewFile(const std::string& path, mate::Arguments* args);
   void CloseFilePreview();
+  void SetGTKDarkThemeEnabled(bool use_dark_theme);
 
   // Public getters of NativeWindow.
   v8::Local<v8::Value> GetContentView() const;

shell/browser/native_window.h

@@ -228,6 +228,8 @@ class NativeWindow : public base::SupportsUserData,
                            const std::string& display_name);
   virtual void CloseFilePreview();
 
+  virtual void SetGTKDarkThemeEnabled(bool use_dark_theme) = 0;
+
   // Converts between content bounds and window bounds.
   virtual gfx::Rect ContentBoundsToWindowBounds(
       const gfx::Rect& bounds) const = 0;

shell/browser/native_window_mac.h

@@ -137,6 +137,7 @@ class NativeWindowMac : public NativeWindow {
       const std::vector<mate::PersistentDictionary>& items) override;
   void RefreshTouchBarItem(const std::string& item_id) override;
   void SetEscapeTouchBarItem(const mate::PersistentDictionary& item) override;
+  void SetGTKDarkThemeEnabled(bool use_dark_theme) override {}
 
   gfx::Rect ContentBoundsToWindowBounds(const gfx::Rect& bounds) const override;
   gfx::Rect WindowBoundsToContentBounds(const gfx::Rect& bounds) const override;

shell/browser/native_window_views.cc

@@ -214,12 +214,7 @@ NativeWindowViews::NativeWindowViews(const mate::Dictionary& options,
   // Set _GTK_THEME_VARIANT to dark if we have "dark-theme" option set.
   bool use_dark_theme = false;
   if (options.Get(options::kDarkTheme, &use_dark_theme) && use_dark_theme) {
-    XDisplay* xdisplay = gfx::GetXDisplay();
-    XChangeProperty(xdisplay, GetAcceleratedWidget(),
-                    XInternAtom(xdisplay, "_GTK_THEME_VARIANT", x11::False),
-                    XInternAtom(xdisplay, "UTF8_STRING", x11::False), 8,
-                    PropModeReplace,
-                    reinterpret_cast<const unsigned char*>("dark"), 4);
+    SetGTKDarkThemeEnabled(use_dark_theme);
   }
 
   // Before the window is mapped the SetWMSpecState can not work, so we have
@@ -330,6 +325,25 @@ NativeWindowViews::~NativeWindowViews() {
 #endif
 }
 
+void NativeWindowViews::SetGTKDarkThemeEnabled(bool use_dark_theme) {
+#if defined(USE_X11)
+  XDisplay* xdisplay = gfx::GetXDisplay();
+  if (use_dark_theme) {
+    XChangeProperty(xdisplay, GetAcceleratedWidget(),
+                    XInternAtom(xdisplay, "_GTK_THEME_VARIANT", x11::False),
+                    XInternAtom(xdisplay, "UTF8_STRING", x11::False), 8,
+                    PropModeReplace,
+                    reinterpret_cast<const unsigned char*>("dark"), 4);
+  } else {
+    XChangeProperty(xdisplay, GetAcceleratedWidget(),
+                    XInternAtom(xdisplay, "_GTK_THEME_VARIANT", x11::False),
+                    XInternAtom(xdisplay, "UTF8_STRING", x11::False), 8,
+                    PropModeReplace,
+                    reinterpret_cast<const unsigned char*>("light"), 5);
+  }
+#endif
+}
+
 void NativeWindowViews::SetContentView(views::View* view) {
   if (content_view()) {
     root_view_->RemoveChildView(content_view());

shell/browser/native_window_views.h

@@ -125,6 +125,8 @@ class NativeWindowViews : public NativeWindow,
 
   bool IsVisibleOnAllWorkspaces() override;
 
+  void SetGTKDarkThemeEnabled(bool use_dark_theme) override;
+
   content::DesktopMediaID GetDesktopMediaID() const override;
   gfx::AcceleratedWidget GetAcceleratedWidget() const override;
   NativeWindowHandle GetNativeWindowHandle() const override;