Is it possible to switch to a new theme without going through the Admin panel? I'm working in a test site (to preserve the existing version). I've uploaded a new theme into the themes directory, but now I need to be able to assign it to my site without going through Settings > Themes.
Currently, you must go to the admin panel to switch a theme, however, you may be able to write a plugin that can switch a theme without having to goto the admin panel. I don't quite understand your particular use case. Are you trying to run two Omeka sites from the same database, so your test site is using the data from your production site? If so, you cannot switch themes this way, because the current theme is specified in the database. Using the Admin panel changes the current theme in the database.
Does this answer your question?