ControlTemplate no longer finds resources

First I created UserControl

.
In xaml I can link to StaticResource

which are in the main app resources.

Then I refactored this UserControl

on Control

and through Themes /Generic.xaml ,
I apply ControlTemplate

(I need this ).

But this way is StaticResource

no longer found (a is XamlParseException

thrown at runtime).

I could combine ResourceDictionaries

that contain StaticResource

, I also need in the Genericl.xaml file, but I would like to avoid this due to performance issues

What is the mechanics of this behavior?