How to apply dynamic theme

specifying theme and skin preferences in page declarations and configuration files, you can apply themes programmatically. You can set both page themes and style sheet themes programmatically; however, the procedure for applying each type of theme is different.

protected void Page_PreInit(object sender, EventArgs e) {     switch (Request.QueryString["theme"])     {         case "Blue":             Page.Theme = "BlueTheme";             break;         case "Pink":             Page.Theme = "PinkTheme";             break;     } }

http://www.codeproject.com/KB/aspnet/dynamicThemes.aspx