How to set special theme for Primary Site

Hi WPMUdev team,

I'm trying to wrap my head around how Premium Theme feature works.

If I understand right I theme has to be Network Disabled and then selected in ProSites > Premium Theme and associated with a level. This makes sense.

If a theme is set to Network Enabled then it shows up in all the sub-sites, which I don't want any themes to be available like this.

The problem I'm having is that my Primary Site needs to have it's own theme. This theme can't be available to any of the sub-sites.

How would I have all my sub-sites use a premium theme and my Primary Site use it's own theme not available to any of the sub-sites?