DetailedList doesn't honor `color` or `background-color` styles

### Describe the bug

The DetailedList widget doesn't honor the `color` or `background-color` style directives.

### How to reproduce the bug

Modify the `examples/detailedlist` example so that the `self.dl = DetailedList(...)` widget created around L103 has colour-modifying style options (see example code below).

### Minimum example code

```shell
self.dl = DetailedList(
...
flex=1,
color="purple",
background_color="yellow",
)
```

### Environment details

- Operating system and version: All
- Python version: all (tested on 3.14)
- Software versions:
- Toga: 0.5.4+

### Additional context

A separate fix will be needed for each supported platform. The fix will involve:

1. Modifying the draw mechanism for the DetailedList widget to honour the current `color` and `background-color` style setting for the DetailedList widget (falling back to appropriate platform default colours)
2. Ensuring that `set_color` and `set_background_color` methods on DetailedList propagate onto the widgets
3. Enabling the `test_background_color`, `test_background_color_reset`, `test_color`, and `test_color_reset` tests for DetailedList in the testbed.
4. Ensuring that the probe backend for DetailedList has a mechanism to extract the current text and background color (and thus the testbed tests pass).

Start with the DetailedList construction around L103 in examples/detailedlist, then trace its draw mechanism and set_color/set_background_color behavior on each supported platform. Enable test_background_color, test_background_color_reset, test_color, and test_color_reset in the DetailedList testbed, and inspect the probe backend's color extraction. Done means the tests pass with explicit styles and after resetting them.

Written by the indexing model from the issue text.