There are two modes that this control uses
1. TreeView - where RadTreeView control is used. Generally you cannot make changes the way of the RadTreeView nodes rendering. You can create some custom classes or wrap the control inside a table.
2. OptgroupDropDownList - this is a custom control that inherits from DropDownList. You can change the render by overriding RenderContents
All the best,
the Telerik team
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