This would usually indicates that the Telerik.Windows.Themes.Expression_Dark.dll is not added in the projects references or it is added but the assemblie is no_xaml and therefore the xaml you need to display the theme is missing.
If that is not the case could you please give us more information on what version of the controls you are using?
Also a simple project could be helpful just to make sure that we are doing the same thing.
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