Thank you for using Sitefinity service.
The backend pages are handled identically to front end pages. The difference is the backend templates aren't exposed by the built in widget template editor/creator. You can access backend template for backend widgets in the SDK which you can download here. With your template added to your project you can reference it on the widget by going to Administration > Backend Pages, finding the correct Page, and then Clicking "edit" on the widget. From there, as in the front end, you will see the "Layout Template Path" field. Insert the path to your custom template.
I hope this helps!
the Telerik team
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