WordPress MU

I've been trying to edit/replace the opening page of my MU website, which I take to be home.php. And yet there is no home.php in my Default theme directory. But there is a home.php in the Home theme directory, which I didn't think I was using.

Any idea what's going on?

Thanks.

By default the theme for the primary site is the home theme.

Thank you.

So I am sure this SHOULD be a simple thing, but how do I change the default theme.
What I am trying to do is use theme for default main blog, and all others, which has typical sidebar/widgets as most all other wordpres themes use, and I can not figure out how to change the main theme to anything other than the home one.

Site Admin -> Themes.

you have to enable themes to have them even show up under the Design tab.