How can I set default theme for all users? I don't want to set it up in web.config. I tried to login as a admin and on breadcrumb menu I changed "Switch Scope (current = user)" to "Switch Scope (current = Shared)"  , but it didn't work. Any idea?

Thanks in advance

Can't you set it in the pages tag in your web.config like this:

<pages styleSheetTheme="Paper">