Stark is a core theme that demonstrates Drupal's default markup. It does not contain any template or css files. This means it uses the templates and css files from Drupal (core) modules.
To avoid obscuring CSS added to the page by Drupal or a contrib module, the Stark theme itself has no styling, except just enough CSS to arrange the page in a traditional "header, sidebars, content, and footer" layout. Thanks to the Drupal 8 Mobile Initiative, Stark uses a responsive layout in Drupal 8. The css rules for this responsive layout can be found in
In Drupal 7, Stark contained css for the layout, the regions of the page. At first, the Stark layout was made responsive (Make Stark use a responsive layout), but the css for that has been removed (Remove all visual from stark). This way, Stark now only outputs html and css from modules.
In a nutshell, Stark outputs vanilla Drupal css and html.