I am a developer working on my first Shopify App. I am using Script Tags to add a small widget above the Checkout button in the Cart Page of a Shopify store. I have been able to do this successfully by running the script tag and adding HTML in one specific Theme. Eventually however, this app could be installed on many different Stores with different Themes.
Since these themes have different html structures my widget will not necessarily fit into the layout the same way (or at all) in different themes.
So my question is how can I support multiple Themes with my App? Is there a standard approach to this?
A related question is, do Shopify Apps that add components to the layout of a Store's pages normally support only a specific list of Themes? Which are handled by the app on a case-by-case basis?