App.cs

public void ShowForm(UIApplication uiapp)
        {
#if NET46
            IntPtr owner = System.Diagnostics.Process.GetCurrentProcess().MainWindowHandle;
#endif
#if !NET46
            IntPtr owner = uiapp.MainWindowHandle;
#endif

            // If we do not have a dialog yet, create and show it
            if (applicationView == null || applicationView.Visibility != System.Windows.Visibility.Hidden)
            {
                // A new handler to handle request posting by the dialog
                RequestHandler handler = new RequestHandler();
                m_Handler = handler;
                // External Event for the dialog to use (to post requests)
                ExternalEvent exEvent = ExternalEvent.Create(handler);
                m_ExEvent = exEvent;
                // We give the objects to the new dialog;
                // The dialog becomes the owner responsible fore disposing them, eventually.
                
                ApplicationView settings = new ApplicationView(exEvent, handler);
                settings.Height = 800;
                settings.Width = 800;
                settings.Title = "Family Browser";
                settings.WindowStartupLocation = WindowStartupLocation.CenterScreen;
                settings.WindowStyle = WindowStyle.ToolWindow;
                settings.ResizeMode = ResizeMode.CanResizeWithGrip;
                settings.MinHeight = 600;
                settings.MinWidth = 650;
                settings.ShowInTaskbar = false;
                WindowInteropHelper x = new WindowInteropHelper(settings);
                x.Owner = owner;
                applicationView = settings;
                settings.Show();
            }
            else if (applicationView.Visibility == System.Windows.Visibility.Hidden)
            {
                applicationView.Visibility = System.Windows.Visibility.Visible;
            }
        }

App_modified_1.cs

ApplicationView settings = new ApplicationView(exEvent, handler);
                ResourceDictionary myresourcedictionary = new ResourceDictionary
                {
                    Source =
                    new Uri("pack://application:,,,/MahApps.Metro;component/Styles/Controls.xaml",
                        UriKind.RelativeOrAbsolute)
                };
                ResourceDictionary myresourcedictionary1 = new ResourceDictionary
                {
                    Source =
                    new Uri("pack://application:,,,/MahApps.Metro;component/Styles/Fonts.xaml",
                        UriKind.RelativeOrAbsolute)
                };
                ResourceDictionary myresourcedictionary2 = new ResourceDictionary
                {
                    Source =
                    new Uri("pack://application:,,,/MahApps.Metro;component/Styles/Themes/Light.Blue.xaml",
                        UriKind.RelativeOrAbsolute)
                };
                //Collection<ResourceDictionary> resourceDictionaries = new Collection<ResourceDictionary>();
                //resourceDictionaries.Add(myresourcedictionary);
                //resourceDictionaries.Add(myresourcedictionary1);
                //resourceDictionaries.Add(myresourcedictionary2);
                //settings.Resources.MergedDictionaries = resourceDictionaries;
                settings.Resources = myresourcedictionary;
                settings.Resources = myresourcedictionary1;
                settings.Resources = myresourcedictionary2;