Refer:
Personalization and User Profiles in ASP.NET[
^]
ASP.NET personalization features include
user profiles, themes, error pages,
and lots more.
How to: Define ASP.NET Page Themes[
^]
Define page themes which you can then apply to one or more pages in your application
ASP.NET Themes and Skins[
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A theme is a collection of property settings that allow you to define the look of pages and controls, and then apply the look consistently across pages in a Web application, across an entire Web application, or across all Web applications on a server.