Problem/Motivation
As detailed in #3050389: Remove dependency to Classy from core themes, Classy will be moved to contrib befire Drupal 10. Before that, we have to remove dependencies on Classy from all core themes.
Part of this process is creating theme-specific copies of any templates and CSS files that were previously inherited from Classy. There should be an easy way to distinguish these used-to-be-Classy templates/CSS files from ones created specifically for a theme.
Proposed resolution
In the templates & css directory for every core theme subtheming Classy, add a classy directory with a README explaining that:
- It is for templates/css copied from Classy,
- If a template in this directory is altered it should be moved outside the classy directory as it's now theme-specific.