Changing theme of whole website at run time

Configuration configuration = WebConfigurationManager.OpenWebConfiguration("~");

System.Web.Configuration.PagesSection appSettingsSection = (System.Web.Configuration.PagesSection)configuration.GetSection("system.web/pages");

if (appSettingsSection != null)

{

appSettingsSection.Theme = "First";

configuration.Save();

Response.Redirect("Default.aspx");

}