Thank you for your question.
To achieve that you will have to use a custom theme for the Docking control.
Attached I am sending you a sample project in which a custom theme is applied. For base it is used the default one Office_Black. Please mind for the "NOTE:"
comments which indicate the place where you have to make changes.
If you have further questions please let us know.
All the best,
the Telerik team
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