Multisite 'global themes' and site specific themes

Hi

I have Pro Sites set up correctly and have also setup the Themes correctly so that a user can select and activate a free theme from one of those made available for the network.

However, if a customer pays extra for a Premium theme. I'd like to make it available to that customer ONLY. Is this possible?

So for example, if i had 200 sites running off the multisite, with 3 free themes available for all the sites, but one of them wanted a special bespoke theme - I'd like to be able to do that for that individual site, but obviously hide it from all the other 199 sites.