There is no need to use the App_Themes folder or any part of the ASP.NET theming mechanism. You can copy your files to a regular folder and follow the instructions for registering the skin with the RadSkinManager in the following help article (which you seem to have already looked at, but I am not sure if you saw the full content):
(see the first part of Using RadSkinManager for applying custom skins
However, note that if you register the skins this way, they will not appear
in the Skin Chooser. To get them to show, you need to embed them in an assembly (see the second part of the Using RadSkinManager for applying custom skins
in the topic linked above).
Once you have the skin correctly registered and showing in the chooser, it will work fine with any RadControl that you provided skin files for.
You have opened a support ticket concerning the same issue. In order to avoid duplicate posts,
I suggest that we continue the communication in the support ticket.
the Telerik team
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