Backstage theme problem

Hello

i figured out small problem with themes on backstage

configuration is as follow

<Window>
<Dockpanel>
<userControl - Ribbon> --> defined in external userControl
<Docksite>
</DockPanel>
</Window>

i set the default theme for window and docksite as in one of your examples
this works perfect Window, Ribbon, docksite and all controls are themed correctly

When i fire application menu for first time Backstage has proper theme also

however when i open it for second time its theme is set to default
taking into account that everything works perfect when i open it for first time
i assume it's some strange bug