fixes [Unable to adjust the color for "Cancel" and "Ok" button in datePicker dialog](https://github.com/flutter/flutter/issues/127739)

### Code sample

<details> 
<summary>expand to view the code sample</summary> 

```dart
import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        useMaterial3: true,
        datePickerTheme: DatePickerThemeData(
          cancelButtonStyle: TextButton.styleFrom(
            shape: const RoundedRectangleBorder(
              borderRadius: BorderRadius.all(Radius.circular(16)),
              side: BorderSide(color: Colors.red),
            ),
            backgroundColor: Colors.white,
            foregroundColor: Colors.red,
            elevation: 3,
            shadowColor: Colors.red,
          ),
          confirmButtonStyle: TextButton.styleFrom(
            shape: const RoundedRectangleBorder(
              borderRadius: BorderRadius.all(Radius.circular(16)),
            ),
            backgroundColor: Colors.green[700],
            foregroundColor: Colors.white,
            elevation: 3,
            shadowColor: Colors.green[700],
          ),
        ),
      ),
      home: const Example(),
    );
  }
}

class Example extends StatelessWidget {
  const Example({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: DatePickerDialog(
          initialDate: DateTime.now(),
          firstDate: DateTime(2020),
          lastDate: DateTime(2030),
        ),
      ),
    );
  }
}
``` 

</details>

### Before 

Not possible to customize action buttons from the `DatePickerThemeData`.

### After 

![Screenshot 2023-08-18 at 16 42 00](https://github.com/flutter/flutter/assets/48603081/4ec01e93-c661-491d-9253-d687da8b76f3)