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In order to change the color of the border of a RadRibbonBarGroup (formerly called RadRibbonBarChunk), you can use the Edit UI Elements dialog. However, it is important to open this dialog by selecting the RadRibbonBarGroup and clicking on the Smart Tag of the Group so that the Edit UI Elements dialog is opened for
I have attached two screenshots that should help you to achieve your goal.
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the Telerik team