We have a known issue concerning the initial theming of RadTextBox at design-time. However, it is working correctly at run-time and you should experience no difficulties. In addition, if you close and then reopen the Visual Studio instance with the solution, the theme will be applied at design as well.
Dan, I am not aware of an issue related to where the RadTextBox is in the controls hierarchy. Therefore, please open a new support ticket and send me a sample application which demonstrates the behavior.
I am attaching my sample project to demonstrate the correct behavior at run-time.
the Telerik team
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