Hi! What kind of membership site are you creating, and what will members be able to do vs non-members? Since Publii is a static site generator, there may be things that you wish to achieve that simply cannot be done with HTML, CSS, and Javascript and require some kind of backend.
As far as I know, there are no themes for membership sites, but all the themes in the Marketplace can be edited to suit anyone’s needs, so long as you have the skills and understand the dev docs. The theme is less of a concern than what features you’re hoping to have in your membership site.
I’d recommend giving this post a look: https://forum.getpublii.com/topic/protected-areas/ In the post, I highlight some services and libraries that could be leveraged for simple authentication/protected pages in Publii.
