Set a single theme for all stores on a marketpress multisite installation

Hi!

The title says it all, pretty much: I want to have a single theme for all the stores on my network, I don't want my users to be able to change it -think of it as what we usually see when creating a store in a site like eBay or Amazon: the structure and the looks of the stores are the same for every user, they can't change that.

I presume the only thing I have to do to achieve that is to set only one of those themes available in marketpress network admin settings as accessible and the other ones as unaccessible, right? If not, how can I do what I want?

Thanks.