How to customize scroll bar for diagram

I would like to customize the scroll bar of the diagram as shown below:
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I was able to customize scrollbar of the Standard ListBox by modifying its control template. I would like to do the same with the diagram but wasn’t able to do. Any advice on how to do it.

Thanks, in advance.

What was the problem that you ran into?

The standard ControlTemplate for Diagram is shown in the file: docs/Generic…XAML.
This is the (renamed) file that is actually used when we compile our DLL.
You can adapt it for your own customization.