In Shopify, there are two types of themes:
- Published (the one that is live and your visitors will see)
- Unpublished (the one that is not live and your visitors won't see, only you). It's useful when you're adding or testing features and you don't want your visitors to see it.
The published theme is always the one that shows on top. You can have only 1 published theme.
Unpublished themes are all the other ones. You can have up to 19.
To make an "unpublished" theme "published", you just need to click on actions > publish
Finally, you can temporarily share unpublished themes with a developer or a friend. Maybe you're testing something and would like an opinion, or whatever the reason may be.
Here's a tutorial of how you can temporarily share an unpublished theme, if you ever need to do so.
I hope this helps to bring you some clarity on the matter.