Please review the following KB article that lists the known reasons for such problem: The styles of the web report viewer's toolbar are messed up
. The only thing that is not mentioned there, since it is a recent finding is if you are targeting .NET framework 4 in your project, the preCondition attribute of the http handler should target v.4.0:
"Telerik.ReportViewer.WebForms.HttpHandler, Telerik.ReportViewer.WebForms, Version=188.8.131.523, Culture=neutral, PublicKeyToken=a9d7983dfcc261be"
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