Stark theme

The Stark theme is provided for demonstration purposes; it uses Drupal’s default HTML markup and CSS styles. It can be used as a troubleshooting tool to determine whether module-related CSS and JavaScript are interfering with a more complex theme, and can be used by designers interested in studying Drupal’s default markup without the interference of changes commonly made by more complex themes.

Users should not use it as a base theme as future core updates may break your theme. Instead, D8 core supplies Stable and Classy. Stable, similar to Stark, has core's templates moved to the theme which will provide stability throughout D8's life cycle. There are also numerous contributed themes any of which are a much better starting point.