Meteor is very lightweight for Shopify stores. The following app-specific files are created/updated whenever a theme is selected via the Themes page in the app admin:

In addition to the app-specific files listed above, the app will also add a Liquid include tag in layout/theme.liquid to load the app's snippet. This include tag is normally injected on a new line right above {{ content_for_header }}.

Do not edit any of the app's files - it will only end in your bitter disappointment (they get overwritten when you make changes in the app).

If you’re looking to customize the app, you’ll want to take a look at using the app's CSS Editor or JavaScript API.