Classy

The Classy theme is available in Drupal 8 and 9 and will be removed in Drupal 10 in favor of the Starterkit theme, which provides the same benefits as Classy while addressing several of the drawbacks.

We do not recommend new projects use this theme, but this existing projects that depend on Classy may use this for Drupal 10+ and it should work no differently than if it were using the Drupal core Classy.

To prepare for migration, this can be safely installed in a version of Drupal with Classy still in core, be it Drupal 9 or a Drupal 10 branch where Classy hasn't yet been removed. If the two themes are present, there won't be a conflict, and Drupal will automatically use the version in the core/ directory.

Configuring a theme to use this contrib version of Classy as its base theme:
In your theme's info.yml

1. Set classy as the base themes (i.e. for themes already using core Classy as a base theme, nothing needs to change)
   base theme: classy
2. Add this contrib theme as a dependency

      dependencies:
        - classy:classy