In Blend you can edit only the default theme (Office_Black). You can find attached an example with a custom theme (Windows7) for the RadUpload control. In case you want to customize another one from the default themes, you can find its project in the "Themes" folder under the root installation folder of RadControls.
Let me know if I can help with anything else.
Greetings,
Dimitrina
the Telerik team
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