Hello Matthias Schuster,
Please excuse us for the delayed response. We had an issue with our site, related to the "RadForms for WinForms" section, which did not alerts us when new tickets have arrived. The issue is now addressed and I am ready to answer your question.
Generally, you should not have any issues with the RadTitleBarVista theme file regarding the steps that you have followed. Since you have your theme's Build Action set to Embedded Resource, the other possible reason for the observed behavior is that you have a typo in your RootNamespace defining the path for the theme resource file.
I am attaching a sample project made according to the steps you described. As you will notice, the Vista theme is correctly shown runtime.
In addition, I want to inform you that in Q1 2009 we included the Vista theme for RadTitleBar in our Vista theme component. So, you do not need to use the theme provided in the Examples, but you can simply drag the Vista component and use its RadTitleBar theme.
I hope this helps. If you have additional questions, feel free to contact me.
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.