Currently there are two options to achieve this. The first one is to predefine the template and the second one - to find the element in the visual tree at run-time and set its background property. That is how the framework works and unfortunately it is impossible for us to expose every single brush through a property, as this will severely damage our API's usability. However, with Q1 SP, we will introduce an implicit-style-based theming mechanism that would make the things easier in your scenario, because you would have direct access to the resources.
All the best,
the Telerik team
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