[REFACTOR] Simplify desktop theme tokens and application rules

[ksamaschke]

Phase

REFACTOR

## Deliverable
Clean up theme tokens and application rules after GREEN while keeping the preserved desktop theme behavior intact.

## Dependencies
- Parent work item: #219.

## Completion evidence
- Cleanup notes for theme tokens and application rules.

• Evidence that the GREEN theme behavior remains intact.

  Acceptance criteria

  • Theme rules are easier to maintain.

• The cleanup does not regress desktop-native theming behavior.

  Parent: Work Item: Preserve theme mode, accent, and background customization within the desktop shell #219