The light blue background comes from the focus and once the RadTileViewItem loses it the background is changed to light gray. You can keep this background if you edit the default template of the RadTreeViewItem - in it there's a VisualState named SelectedUnfocused and all you have to do is change its TargetName from SelectionUnfocusedVisual to SelectionVisual (after that you can delete the Border with name SelectionUnfocusedVisual because it's not used anymore).
For further references please examine the attached project and if you have more questions feel free to ask.
All the best,
the Telerik team
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