Tree does not implement styles

### Describe the bug

Working on adding a dark mode to an application. All widgets except tree can be restyled with color and background color.

### Steps to reproduce

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

### Expected behavior

Tree control should allow changing color and background color.

### Screenshots


### Environment

- Operating System: Linux Mint 22 (based on Ubuntu Noble)
- Python version: 3.12.3
- Software versions:
- Briefcase: 0.3.20
- Toga: 0.4.8
- toga-gtk: 0.4.7 (in pyproject.toml)

### Logs

```

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### Additional context

_No response_

Start by reproducing the Tree control styling behavior with Toga 0.4.8 and toga-gtk 0.4.7 on Linux. Trace the Tree control's styling entry point and verify that setting color and background color takes effect; done when both styles work consistently with the other widgets.

Written by the indexing model from the issue text.