The difference between the RadControls for ASP.NET Ajax in Sitefinity 3.5 and 3.7 is that the skins have been moved to the assemblies and are now loaded with as webresource. It seems that there have been changes in the skin itself between the different versions of the RadControls. What you can do is if you have your old CSS files with the skins you set the RadGrid's property EnableEmbeddedSkins to false and then put your old css in the theme.
All the best,
the Telerik team
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