Hello UnkleFrank,
Thank you for your question about sharing the staging environment. At the moment, there is not a feature that will allow your clients to refresh on/off certain themes.
Alternatively, you can provide your clients a user account that will allow them to see the staging environment. Or, you could copy/clone your site to a public development site that they could access from their location. However, this would still display one theme at a time unless you installed multiple development sites with a different theme installed in each one.
Best,
Christopher M.