Unfortunately the Background property of RadComboBox is not bound to the control template and cannot be used to change the control background. We have some concerns about it, because it is not obvious which exactly color should be changed - the popup background, the textbox background in editable mode, the highlighted element background, etc. There are too many backgrounds to be set and one property is not enough, hence it is not bound.
To change the colors you will have to modify the control theme. Please, find attached a simple project that demonstrates how to change the theme of RadComboBox and that can be opened in Blend. I included the default and the Office themes of RadComboBox.
I apologize for the inconvenience, the Blend support and the customizability of the themes are not on the required level of quality, but we understand that they are very important and most probably very soon we will be releasing a new service pack with improved Blend support and additional properties that will allow the developers to change various control colors.
All the best,
the Telerik team
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