A theme can be deleted if it's not used on any published site by an account or content admin.
Themes Library shows the list of the published sites where the theme is being used. Please note that unpublished sites using the theme will not show up on the list.
To delete a theme:
1. From the Themes dashboard, you'll be able to see the number of sites that are using the theme. You can also click on the sites to view them.
2. You won't be able to delete the theme if there's at least one site that is currently using it.
You can also view the sites that are currently using the Theme inside the Theme Editor.
If there is at least one site using the theme, and you still wish to delete it, you have to go to the editor of the site and select a different theme for it. Please refer to this article for more information on How to Change your Theme.
3. If you had to assign a different theme for a site as described on the previous step, then refresh the Themes Library to clear the list of published site/s.
4. Delete button should now be available after clicking More in Themes Library.