A Master Page enables you to share content across multiple pages in a website.
A Theme enables you to control the appearance of the content.
You create a Theme by adding a new folder named App_Themes.
Each folder that you add to the App_Themes folder represents a different Theme.
If the App_Themes folder must be located in the root of your application.
The contents of a Theme folder are compiled into a new class.
Be careful not to name a Theme with a class name that conflicts with an existing class name.