Buddypress:default theme

I want all blogs on my buddypress site to use the same theme – Grid Market.

It is the only one I have network enabled, but when a site is created it defaults to using Twenty Ten. If they choose Grid Market (which is the only other option they have on the theme page, Twenty Ten is no longer available to them.

Is there a way to just say “this is the default theme and all new blogs should start with this one”?