When you move your DotNetNuke 7 instance from Windows Server 2012 to a new Windows Server 2012 R2 instance you might end up with a not properly rendered skin. This issue relates to a situation where the releated Telerik Script Manager is not properly loaded.
Querying the affected website with Web Page Test showed in this case that the query for the Telerik.Web.UI.WebResource.axd resource resulted in an 404 http error.
Searching the web resulted in a variation of proposed solutions to this issue. In this case the solution is as follows.
The Telerik.Web.UI.WebResource.axd handler requires to be added to the system.webServer/handlers section of the web.config.
If the system.webServer node does not have a handlers node, just add it.