Online Customizer themes and versioning
This update introduces a new way to manage your online store customizations via themes!
With themes you can publish your online customizations to specific stores or to all of your stores - however you want. The feature also supports theme drafting, so you can work on themes and save edits before publishing them to your live online store.
Currently existing customizations have been moved to this new theming system. Check out the themes feature under Settings -> Online Store