dynamically apply a Theme to a page

1、You can dynamically apply a Theme to a page by handling the Page PreInit event. This event is the first event that is raised when you request a page. You cannot apply Theme dynamically in a later event such as the Page Load or Pretender events.But You can modify the CssClass property during any event before the page is rendered 

2、Because the control tree has not been created when the PreInit event is raised, you can’t refer to any controls in a page PreInit event.