Themes

hi,

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.
Theme can be applied all two way which is describe by Senthilkumar
but you can also applied in page init event.

protected void Page_PreInit(object sender, EventArgs e)
{
    // Applying theme to a particular page
    Page.Theme = "myTheme";
}

please refer
http://www.dotnetfunda.com/articles/article402-working-with-themes-in-aspnet.aspx
is good article on Theme.
hope this help.

Hi,

If you are looking for assign the single theme to all the asp.net pages then typically across application. Then you can set it in the web.config file.

                    

 If you want to apply for the page wise then you have to add the theme attribute in the page directive.  
<%@ Page Theme="ThemeName" %>


 I suggest you to go through these urls:
 http://msdn.microsoft.com/en-us/library/0yy5hxdk(v=vs.80).aspx
 http://msdn.microsoft.com/en-us/library/ykzx33wh.aspx