[BUG] Popup BackgroundColor should adhere to AppTheme

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Did you read the "Reporting a bug" section on Contributing file?

- [x] I have read the "Reporting a bug" section on Contributing file: https://github.com/CommunityToolkit/Maui/blob/main/CONTRIBUTING.md#reporting-a-bug

### Current Behavior

## Bug
Popups using `DefaultPopupSettings` always default to a hardcoded `White` `BackgroundColor`. If the app is in dark mode, child controls (like `Label`) default their `TextColor` to `White` as well, making the text invisible against the background.

## Current Workaround Issues
Overriding `Options.DefaultPopupSettings.BackgroundColor` does not solve this because there is currently no way to bind `DefaultPopupSettings` to `AppTheme`. As a result, BackgroundColor must be explicitly set on every individual Popup instance via `{AppThemeBinding}`.

### Expected Behavior

The default `BackgroundColor` of popups should be the same as the for any other page. Therefore it adheres to the AppTheme.

### Steps To Reproduce

1. Make sure the devicetheme is in darkmode.
2. Open and run solution from reproduction repository.
3. Click the "Click me"-button.
4. The label of in the shown popup has a white `TextColor`.

### Link to public reproduction project repository

https://github.com/meyerdominik/MauiTest/tree/ToolkitPopupAppTheme

### Environment

```markdown
- .NET MAUI CommunityToolkit: 15.0.1
- OS: Android 15.0
- .NET MAUI: 10.0.100
```

### Anything else?

_No response_

Start by locating the popup implementation and DefaultPopupSettings, then reproduce the issue using the linked MauiTest repository with the device in dark mode. The work is done when a popup using default settings follows AppTheme for its background and its child text remains readable without per-popup overrides.

Written by the indexing model from the issue text.