Is there a plugin that will dedicate a particular theme to only one of my users?

I have a client who's asking for a custom landing page, so they can promote an item for a couple of months. Is there a plugin that will dedicate a particular theme to one of my clients, and non of my other clients will know about this theme?

I'm using the Pro Sites plugin, but I don't see an option that will allow me to do what I need. Maybe there's a plugin that will allow me to accomplish what I'm trying to do.

Thanks