In order to apply these changes you have to edit the ControlTemplate of the RadTreeView in Blend. More information you can find here.
Then you will be able to modify the default style of the RadTreeView to match your requirements.
In order to modify the MouseOver and Selected background color of the RadTreeViewItem you can modify the MouseOver/Selected resource brushes - ControlSubItem_OuterBorder_MouseOver, ControlSubItem_InnerBorder_MouseOver, ControlSubItem_Background_MouseOver and ControlSubItem_OuterBorder_Selected, ControlSubItem_InnerBorder_Selected, ControlSubItem_Background_Selected.
Another approach is to set the new values in the MouseOver and SelectionVisual Borders:
I prepared a sample project illustrating the first approach. Please take a look at it and let me know if it works for you or if you need more info.
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