As of Q1 2014 the WPF ReportViewer uses implicit styles instead of the StyleManager to apply themes. More details how to update your project and how to troubleshoot possible issues that may appear are described in the following forum thread: Upgrade to 2014Q1 Reporting.
The latest available Telerik Reporting release Q1 2014 SP1 version 8.0 14.507 is built against UI for Silverlight Q1 2014 SP1. Thus you will need to download and the corresponding UI for Silverlight hotfix or a greater version. Please keep in mind the only files part of Telerik Reporting are the Telerik.ReportViewer.Silverlight.dll and Telerik.ReportViewer.Silverlight.xaml. The rest files should be updated from the UI for Silverlight product folder. If you use customized themes, you will have to update the Telerik.ReportViewer.Silverlight.xaml file as well.
Details how to add a Silverlight ReportViewer are available in the How to: Add report viewer to a Silverlight application help article.
Once you have the XAML files, you can modify the template of the viewer and make the RadContextMenu with the Rendering Extensions to have transparent background or change the parameters are style.
I hope this helps you.
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