Add an option to have the island always in dark

### Feature Description

Add an option to have the island always in dark (black background) in dark mode and light mode.

### Use Case

To be able to have the island always in black, even in light mode.

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution

_No response_

### Alternatives/Existing Solutions

_No response_

Start by locating the island UI and its dark/light mode handling in the repository; the issue names no file or test. Trace how appearance options are exposed, then verify that the island remains black in both modes across the compositors listed in the issue.

Written by the indexing model from the issue text.