Hi,
You can apply themes to a page, a Web site, or globally. Setting a theme at the Web site level applies styles and skins to all the pages and controls in the site unless you override a theme for an individual page. Setting a theme at the page level applies styles and skins to that page and all its controls.
Refer:
MSDN : ASP.NET Themes and Skins[
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MSDN : How to: Apply ASP.NET Themes[
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Here[
^] you can have a look on small article.
Here[
^] is an article which will demonstrate you "How the set themes from database".
All the best.
--Amit