Can I choose what ever theme I want for Multisite?

I am looking for themes to use as my landingpage/startpage for multisite hosting. Can I for instance find a theme from and use as my site for comunicating, and information on how to sign up for a blog or a site. And I can have support info and videos on these sites? Almoust like a startpage as has? And only when loged in they can see the support videos and text explanations, join a supportforum and buddypressforum? Does any theme work for this or do I need a spesial page as "The homepage theme"?