[Review] Admin theme: the Default Admin adapter and the Media Library pilot

Problem/Motivation

Default Admin is the only admin theme in Drupal 12 core and the reference implementation of the contract. An adapter maps every contract token to its --admin-* variables, so contract-based UI follows its accent color and dark mode. The Media Library pilot replaces its Media Library CSS.

The full brief, with what we built, screenshots and how to try it in the demo site: https://project.pages.drupalcode.org/design_system_api/reviews/admin-theme/

Questions

1. Can the --admin-* variables be treated as stable API?
2. Should Default Admin implement the contract itself instead of through an adapter?
3. Which tokens don't fit, and which variables did we miss?
4. Is anything lost when the pilot replaces the theme's Media Library CSS?
5. Where should adapters for contrib themes (Claro, Gin) live?

Short answers help as much as long reviews ("2: no, because …"). Each question in the brief links to the part of the concept it affects.

Proposed resolution

Collect answers here. Settled answers become decisions, change the concept, and are summarized under "What we heard" in the brief.

Remaining tasks

• Review by maintainers of this area.
• Summarize the outcome in the brief.

Proposing changes to the brief

The brief is docs/reviews/admin-theme.md in this repository: open a merge request from this issue's fork.