Which method is essential for creating a Shopify app that interacts with a theme?

The method of using the Shopify API for development is essential for creating a Shopify app that interacts with a theme because the Shopify API provides the necessary endpoints and functionality to communicate between the app and the Shopify store. The API allows developers to access a wide range of resources, such as products, collections, orders, and themes.

By leveraging the Shopify API, developers can programmatically manage theme assets, make changes to the store's layout, or retrieve information to display in the theme. This interaction is foundational for building dynamic features or integrations that enhance the user experience on a Shopify store. Without utilizing the API, the app would lack the capability to perform foundational tasks that require data manipulation and retrieval from the store’s backend.

In contrast, while third-party frameworks can assist in development, they generally rely on the use of the Shopify API to function effectively with the platform. Custom plugins in JavaScript may enhance theme functionality but would not provide a way to manage or interact with a store's data directly without the API. Lastly, configuring settings within the Shopify admin is important for store management but does not allow for the level of customization and interaction with store themes that the API provides.