Load WPF styles or other Static Resources from an external file or assembly

Solution 1:

Referencing an external ResourceDictionary (XAML File):

<Application.Resources>
    <ResourceDictionary Source="MyResources.xaml" />
</Application.Resources>

Referencing an external ResourceDictionary (DLL):

<Application.Resources>
    <ResourceDictionary Source="/MyExternalAssembly;component/MyResources.xaml" />
</Application.Resources>