[GREEN] Implement desktop shell theme preservation for mode, accent, and background

[ksamaschke]

Phase

GREEN

## Deliverable
Implement the minimum theme-system changes needed to preserve customization in the desktop shell.

## Dependencies
- Parent work item: #219.

## Completion evidence
- Implementation notes for theme application rules in the desktop shell.

• Evidence that the RED tests now pass.

  Acceptance criteria

  • Users can still select mode, accent, and background options.

• The resulting UI still reads as a desktop shell rather than a dashboard.

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