Hi,
You can try to refer this article: Themes for Silverlight Applications.
A theming system for Silverlight 2 apps... this link steps you through creating a set of theme assets, adding customizability of themes, and using them in an application, including selecting a theme dynamically... complete with source code.
Please remember to mark the replies as answers if they help and unmark them if they provide no help.
Yours sincerely,
Amanda Wang
Microsoft Online Community Support