Development themes are temporary, hidden themes that are connected to the Shopify store you’re using for development. When you run
shopify theme serve, Shopify CLI automatically creates a development theme inside the store you’re working on, and will reuse that theme each time
shopify theme serve is subsequently run during the same session. You don’t need to worry about others viewing or making edits to it because development themes are not visible on the Online store > Themes page.
Learn more about Development Themes.