You can find detailed information about the different ways for setting a theme in this help article and to choose the most appropriate for your scenario. In addition, this blog post describes how to change Silverlight application themes at runtime.
With the official 2012 Q1 SP1 release of RadControls we introduced Implicit Styles for our controls, which grant the option to disable the Telerik StyleManager. To get familiar with them, please refer to our help.
I hope this helps.
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get it now >>