I answered your support ticket on the subject and for your convenience I am pasting below my answer:
Thank you for the provided additional information.
The purpose of the Telerik.Web.Design.dll assembly is to separate the design-time code that visualize the control in Design mode of Visual Studion, from the web control implementation code. Telerik.Web.Design.dll is not responsible to the control appearance in runtime.
Since Q3 2011, we also separate the controls skins from the source code in the new Telerik.Web.UI.Skins.dll assembly. Only the default skin is stored in Telerik.Web.UI.dll. You can find more information about this change in the following resources: Separate Skin Assembly in RadControls for ASP.NET AJAX with Q3'11
and Telerik.Web.UI.Skins assembly as of Q2 2011
Since you use a skin different from the Default one, my suggestion is to copy the Telerik.Web.UI.Skins.dll in the bin folder of your project, add a reference to it, compile the project and test the problematic page.
If this does not fix the problem, my recommendation is to register the external CDN skins of Telerik controls as explained in the following help article: CDN Skins overview
If the issue still persists, please, isolate it in a simple working project and send it for examination on our end.
All the best,
the Telerik team
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