Adding a back in stock app
A back-in-stock app lets you notify your customers whenever a product they are interested in is back in stock. This is a great way to reduce buyers' frustration and increase sales.
However, a theme cannot provide a back-in-stock functionality due to what a theme can access in your store. To add such a feature, you will need to use an app.
There are several apps in the app store, but we can recommend the app called Back In Stock. If this app does not match your use case, we recommend you contact Shopify support for more recommendations.