First of all, thanks for the speedy reply Nobster.
So I started from scratch:
New WPF App; referenced WPFToolKit, added your code to constructor window for testing; added fresh "System.Windows.Controls.Theming.ExpressionDark.xaml" to Project.
and.....nothing :) no build for me
Is my assumption correct that I have to alter the ResourceDir of the "Theme".xaml ?
I altered the line xmlns:vsm="clr-namespace:System.Windows;assembly=System.Windows" to
the WPFToolKit assembly but that only led to another row of errors...
What am I missing? :) The WPF Theme section described porting these themes as totally simple :(