Go to solution explorer and right click on the website node.
Choose "Add ASP.NET folder" and click "Theme".
You will see new folder under the App_Themes folder. You will place all the files for that theme on this folder.
Create skin files and dump all css files, images on this folder.
You can create as many themes as you like and easily switch change the theme of your site as per your need. How to apply theme You can configure on the web.config file to apply that theme on the entire site.
Open up the web.config file and add the following entires.
You can also apply the specific themes for pages of specific folder. To do so you have to create web.config on the folder level and override the theme configuration from the web.config of the root. Also you can specifiy theme on page level as well.