With reference to the following link:
If we add the theme attribute to the @ Master directive, the page will raise an error when it runs.
But we can apply themes by using the following approaches:-
1. As master pages is placeholder/templated and it is merged with the content page very early in the page execution life-cycle. The theme that is applied to the the content pageis applied to the master page as well.
2. If the site as a whole is configured to use a theme by including a theme definition in the pages element.
Hence there is no theme attribute in the @Master directive.
Source: From books and MSDN | Asked In: From books and MSDN |