You could create a StyleSelector that alternates the returned style. Add it an EvenStyle and OddStyle properties and every time a SelectStyle is invoked swap the styles. Please check the attached project.
Note that if you are using virtualized RadComboBox you may need to add something like "id" to your view model items since the SelectStyle will be called pretty much at random scrolling up and down. In that case you will be able to return one of the styles if the id%2 == 0 and the other otherwise.
All the best,
the Telerik team
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