Thank you for writing.
You need to change the SignImage property of the desired states. Attached you can find six images based on the one that you sent. Two of them (the one you sent) for the default normal and expanded state, two for the hovered state and two for the selected state - respectfully with blue and yellow arrow. Additionally, you can find a video demonstrating how to change the images for the different states. Lastly, you can find a project and the resulted theme applied to RadTreeView.
I hope that you find this information useful.
the Telerik team