How to apply Theme to all .aspx pages from Master Page

Hi,

     I am working on Themes. Till now I can sussessfully change the Single Page theme when my user logs in.

I Used :

Page.Theme = "RedTheme";

and redirect it.

But if i take master page then i unable to apply the selected theme to all pages. My User can able change theme now. But only for single page. How to apply for all pages?