**Fixes #152846 in accordance with iOS HIG**
Previously the `_CupertinoThemeDefaults _kDefaultTheme` would set the `primaryContrastingColor` to `CupertinoColors.systemBackground`, which was white-ish in light mode, and black-ish in dark mode. That was in accordance with Apple Design Resources from 5 years ago.
> Before:
> <img width="594" alt="image" src="https://github.com/user-attachments/assets/63e88abb-6933-446f-a7ba-55109d0f353c">
As of now, iOS HIG suggests that the `primaryContrastingColor` (in combination with the currently default `primaryColor: CupertinoColors.systemBlue`) be white (regardless of light/dark modes, contrast, elevation, etc.)
> After:
> <img width="594" alt="image" src="https://github.com/user-attachments/assets/2a48f22b-a886-46dd-aada-6d157cb4ac06">
Example code:
```dart
import 'package:flutter/cupertino.dart';
import 'package:flutter/widgets.dart';
void main() =>
runApp(
CupertinoApp(
theme: CupertinoThemeData(
brightness: Brightness.dark,
),
home: Center(child:
CupertinoButton.filled(
onPressed: () {},
child: Row(
mainAxisSize: MainAxisSize.min,
children: <Widget>[
Icon(CupertinoIcons.add),
Text('Add'),
],
),
)
)
)
);
```
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