I need to separate application styles to several xaml files. But I also need to define some shared values like
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Using merged dictionaries can get a bit tricky if you have dependencies between other merged dictionaries.
When you have multiple application-scope resources the order of the declare is important. They are resolved in the inverse order of the declare, so in your case you should have the order.
Also, you may need to reference the other ResourceDictionary in Styles1.xaml. This worked for me in Styles1.xaml.