In order to have the Expression_Dark theme you will have to edit the template that is inside the Telerik.Windows.Controls.Input.xaml resource file responsible for the Expression_Dark theme. You can find this file under your installation folder. It should be something like:
C:\Program Files (x86)\Telerik\RadControls for WPF Q1 2012 SP1\Themes\WPF40\ExpressionDark\Themes
You can also find this file included into the previous project I sent you.
the Telerik team
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