A theme is a collection of files that determines the visual presentation of your store. When you first install Commerce, the design elements of the store are based on the Default Theme. In addition to the initial default theme that comes with your Commerce installation, there is a wide variety of available themes that you can use as is or modify for your needs.
A responsive theme adjusts the page layout to fit the view port of the device. The sample Luma theme has a flexible, responsive layout that can be viewed from the desktop, tablet, or mobile device.