Resources can't be found when App.xaml's Build Action = Page

I changed the build action of my App.xaml to "Page" so I could handle a splash screen and make sure my application run as a single instance only (having my own entry point). It works fine in run-time but in design-time the application cannot see the my resources anymore. Resources are in separate xaml files located in the same project. How can I make my app see the resources in design-time again?

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Gustavo - Central California