Problem/Motivation
Like Classy #3050389: Remove dependency to Classy from core themes, we'll be decoupling Bartik/Seven/Claro/Umami from Stable. This will make it easier for these themes to receive improvements as they are no longer beholden to BC, and will facilitate easier major version updates from Drupal 10 onward.
Proposed resolution
- Create tests that identify which stable templates/css are different from the module code they originate from.
- For each css file/template that differs, determine if the Core theme should receive a copy of Stable's asset, or if it's acceptable for the core theme to use the updated version.
- Provide theme specific copies of these assets where needed.