Themes are simply templates that allow you to define the look of your web forms and controls and they may be utilized in your web application to provide a consistent user interface.  A skin is a template that contains a set of properties used to standardize the various attributes of your web controls.  A theme may consist of a single skin and one or more style sheets.  The benefit of using themes enables your development team to write the code without worrying about the attributes of a button control.  For example, if you have a theme title black, all the development team has to do to maintain a consistent user interface is assign the theme to the control in question.  While themes may not be appropriate for all occasions, they are flexible in that they can be applied to an entire application, web form or even a single control.