if we copy a theme folder on a new Prestashop installation, we can see it in the backend (preferences/themes) but we can't use it (wrong configuration file) because there's no entry of this theme in the database (ps_themes)
If we add manually this entry in the database it works. So this is a bug.
Whether we don't show this theme in the backend because the team doesn't want us to install a theme like this or we can do it, but not the solution we have now.
In the backend, Prestashop shows us a theme without preview or name, because it sets these data with the database