Themes generally refer to the look and feel provided for the UI. This is, indeed, quite separate from the platform because the platform provides a basic HTML structure but the styling of that structure is entirely up to the theme.
The theme files provide the styling for forms and console pages. The theme also provides functionality for console pages and other such pages that aren't forms in the solution.
While there are many ways to accomplish this, Kinetic has decided to move forward with primarily REACT bundles that contain css and some js for the forms as well.