A theme is a visual definition of the look & feel for common controls in an Operating System.
Themes are usually end-user selectable, changing the theme at the OS level, impacts the look & feel of running applications that are ‘theme aware’. As an example, on Windows Vista and Windows 7, a popular/desired theme is the Aero theme.
Link – http://blogs.msdn.com/jaimer/archive/2010/03/30/theming-in-windows-phone.aspx
