fix: dark mode on Linux default themeing (#47920)

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Shelley Vohr <shelley.vohr@gmail.com>

shell/browser/ui/electron_desktop_window_tree_host_linux.cc

@@ -25,6 +25,7 @@
 #include "ui/linux/linux_ui.h"
 #include "ui/ozone/public/ozone_platform.h"
 #include "ui/platform_window/platform_window.h"
+#include "ui/platform_window/platform_window_init_properties.h"
 #include "ui/views/widget/desktop_aura/desktop_window_tree_host.h"
 #include "ui/views/widget/desktop_aura/desktop_window_tree_host_linux.h"
 
@@ -307,4 +308,14 @@ void ElectronDesktopWindowTreeHostLinux::DispatchEvent(ui::Event* event) {
   views::DesktopWindowTreeHostLinux::DispatchEvent(event);
 }
 
+void ElectronDesktopWindowTreeHostLinux::AddAdditionalInitProperties(
+    const views::Widget::InitParams& params,
+    ui::PlatformWindowInitProperties* properties) {
+  views::DesktopWindowTreeHostLinux::AddAdditionalInitProperties(params,
+                                                                 properties);
+  const auto* linux_ui_theme = ui::LinuxUiTheme::GetForProfile(nullptr);
+  properties->prefer_dark_theme =
+      linux_ui_theme && linux_ui_theme->PreferDarkTheme();
+}
+
 }  // namespace electron

shell/browser/ui/electron_desktop_window_tree_host_linux.h

@@ -61,6 +61,9 @@ class ElectronDesktopWindowTreeHostLinux
   // views::DesktopWindowTreeHostLinux:
   void UpdateFrameHints() override;
   void DispatchEvent(ui::Event* event) override;
+  void AddAdditionalInitProperties(
+      const views::Widget::InitParams& params,
+      ui::PlatformWindowInitProperties* properties) override;
 
  private:
   void UpdateWindowState(ui::PlatformWindowState new_state);