You can download the source code of the Telerik UI for WinForms suite directly from your Telerik account: https://docs.telerik.com/devtools/winforms/installation-and-upgrades/download-product-files
There is also a project for VisualStyleBuilder and you can have a look at it.
However, please have in mind that you can use the tool once you have the WinForms suite installed on your computer. The easiest way to start it is to click the Windows button and type 'Visual Style Builder'.
I hope this information helps. If you need any further assistance please don't hesitate to contact me.
Dess | Tech Support Engineer, Sr.
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