You create a Microsoft ASP.NET application by using the Microsoft .NET Framework version 3.5. 
The application uses 10 themes and allows the users to select their themes for the Web page. 
When a user returns to the application， the theme selected by the user is used to display pages in the application. This occurs even if the user returns to log on at a later date or from a different client computer. 
The application runs on different storage types and in different environments. 
You need to store the themes that are selected by the users and retrieve the required theme. 
What should you do？ （）