A theme controls the look and feel of a Shopify online store.
A store can have a maximum of 20 themes, one of which is the published theme that customers see when they visit the online store. Customers can't see unpublished themes. When you publish a theme, the previously published theme becomes unpublished.
What you can do with Theme
The Shopify API lets you do the following with the Theme resource. More detailed versions of these general actions may be available: