Hi John,

At present it is possible to use custom themes within categories, used in conjunction with Audience visibility it should be possible to arrange what you are looking for.

We are looking at developing this kind of functionality further as mentioned here. There's also some discussion here which might be useful. 

There has been a fair amount of thought on true multi-tenancy, and we are very much aware how popular the idea is. However technically it could require a very significant amount of work. 

hope this is useful,

regards,

George.