When you create a new theme there’s a “Theme Starting Point” dropdown. If you choose “Blank Theme” the folder will only contain a themes.json and fields.json file and the fields.json will my empty. If you choose “Boilerplate” you’ll receive a clone of HubSpot’s boilerplate theme which includes all the files for a basic theme setup and the fields.json will have default content in it.
I think the word ‘override’ is a little misleading, not sure why it’s labeled that. It’s basically just a stylesheet for setting all your global styles that use data from the fields.json file.