I know the question is old but can help others, I know two ways to do:
1º Way is Via Nuget: Right click on the project and select "Manage Nuget Packages" option and install Wpfthemes.
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It will command the WPF.Themes reference automatically
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Now in your main form do the same as the image below
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Note that this way the theme will only be applied when the project is running
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2º Way: Downloading the Thema code on the link: https://wpfthemes.codeplex.com/SourceControl/latest
Add the downloaded Thema file to your project and within the app.xaml add the reference to it:
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Note that this way it is loaded in design mode and you have access to the code to edit according to your preference.
Obj: If it doesn’t appear first, from a rebuild in your solution, close and open the visual studio.
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Have you checked if WPF is not pointing to the Client Profile instead of pointing to your . NET Framework? For example, if your library points to the . NET 3.5, make sure WPF is also pointing to . NET 3.5 or 4.0, not Client Profile.
– Eduardo Silva