In order to achieve this you have to edit the style of the RadComboBoxItem as well. You can easily do this with Expression Blend. Just drop a ComboBox control on the design surface, right-click it and choose from the menu Edit Template->Edit a Copy
. Do the same for the RadComboBoxItem. Then Blend will generate all the needed resources.
There find the Border with x:Name="HighlightVisual"
. In it there is another Border of which you have to set the CornerRadius to SplitButton_SpanCornerRadius
Hope this helps.
All the best,
the Telerik team
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