There are no exposed properties for the visual states in particular. Our theming mechanism is build in such a way that it tries to keep consistency across all controls. That is why controls such as RadComboBox, RadButton, RadNumericUpDown, etc. use a ButtonChrome control in their control templates. The ButtonChrome is responsible for rendering mouse over brushes, pressed and selection brushes, etc.
To change the mouse over color of RadComboBox, you need to apply a Style for the ButtonChrome. Attached is a short movie that shows how to do that. Please, note that after you generate the style for ButtonChrome, you have to make sure all your brushes reside at the top of the page, before any other Styles that reference them.
Although the movie shows how to change the mouse over for the Silverlight version of RadComboBox, the approach is the same for the WPF version.
I hope this helps.
the Telerik team
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