[OOBE]SetTheme function added for oobe in HandleThemeChange function. (#28345)

src/settings-ui/Settings.UI/SettingsXAML/App.xaml.cs

@@ -288,6 +288,7 @@ namespace Microsoft.PowerToys.Settings.UI
                 {
                     var hWnd = WinRT.Interop.WindowNative.GetWindowHandle(oobeWindow);
                     ThemeHelpers.SetImmersiveDarkMode(hWnd, isDark);
+                    oobeWindow.SetTheme(isDark);
                     SetContentTheme(isDark, oobeWindow);
                 }
 

src/settings-ui/Settings.UI/SettingsXAML/OobeWindow.xaml.cs

@@ -133,7 +133,7 @@ namespace Microsoft.PowerToys.Settings.UI
             }
         }
 
-        private void SetTheme(bool isDark)
+        public void SetTheme(bool isDark)
         {
             shellPage.RequestedTheme = isDark ? ElementTheme.Dark : ElementTheme.Light;
         }