You can activate the app in any of the installed themes in your store.
– The themes which you have installed in your store will be listed in this option and you can activate the app in any one from that installed theme.
For ex: If you are looking to test the app in the unpublished theme instead of live theme then you can select any one of the unpublished themes and test the app.
Note: If you activate the theme in unpublished theme then it will not be shown in the live theme the theme which is selected in this option, the app will show for that theme only.