base theme: stable or when you don't define a base theme.
Stable was introduced in Drupal 8 as a backwards compatibility layer for Drupal 8's core markup, CSS and JS. When no
base theme is provided by a theme, stable will automatically be used.
This change allows core markup, CSS and JS to further evolve throughout Drupal 8, while still allowing themes to have a stable base for the clean, minimal markup provided by Drupal 8 core. This is especially useful for front-end developers who would prefer to add in their own classes to markup where needed, rather than remove Drupal's default classes. Front-end framework implementations like Bootstrap or Foundation might find this particularly useful.
To disable backwards compatibility, a theme can still directly extend core by setting the base theme to
base theme: false
- The official change record: Stable base theme added as default for backwards compatibility