The thmx file extension is associated with Microsoft Office 2007/2010 themes.The themes are stored in *.thmx files, and the built-in Office themes are stored this way in C:\Program Files\Microsoft Office\Document Themes 12\.

The format of a thmx file itself is a ZIP-based format (just like the OpenXML file format).
With the use of the thmx templates, it is possible for users to create uniform looking documents in Word (also in Excel and PowerPoint).

Workaround: Currently, exporting of RadDocument style sheet is supported only to custom XAML format (http://docs.telerik.com/devtools/wpf/controls/radrichtextbox/features/styles/style-sheets ).