Themes included in Drupal 8
When you install Drupal 8, there are a three themes that are included. These can be classified as:
Accessible via the UI
Base themes
Accessible via the UI
These themes are accessible via the user interface.
Bartik
The default Drupal frontend theme, this will be active for all content pages when Drupal is initially installed. It is in fact a subtheme of Classy—that is, it is based on or "built on top of" the Classy theme.
Seven
This is the default backend/admin theme. This can also be set as the default frontend theme, but it is really designed to be the common administration theme. This too is a subtheme of Classy.
Stark
This is a very minimal theme that includes the smallest amount of HTML markup and styling. Unlike Bartik and Seven, this theme is not built on top of anything and shows off the raw HTML markup that comes out of modules. As such, it's useful to developers to determine whether module-related CSS and JavaScript are interfering with a more complex theme.
