[RED] Capture failing tests for desktop shell theme preservation

[ksamaschke]

Phase

RED

## Deliverable
Create failing tests that assert mode, accent, and background customization remain available while the desktop shell retains its native structure.

## Dependencies
- Parent work item: #219.

• Blocked by Work Item: Replace dashboard landing with desktop shell frame #211.

• Blocked by Work Item: Move app preferences and theme controls into dedicated Settings window #213.

  Completion evidence

  • Failing tests naming preserved theme controls and desktop-shell constraints.

• Coverage references for non-default theme combinations.

  Acceptance criteria

  • Tests fail until the redesigned shell preserves themeability correctly.

• The relationship between theming and desktop structure is explicit enough for implementation.

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