[MaironLucas]

• Add properties to DatePickerThemeData:
  • toggleModeStyle allow customization on _DatePickerModeToggleButton TextStyle;
  • toggleModeForegroundColor allow changing the color of both button and Icon with consistency;
• Compatible with existing code by the use of defaults;
• Adjust test to cover new properties

How to customize toggle mode style before:

  showDatePicker(
    context: context,
    firstDate: DateTime(2025),
    lastDate: DateTime(2026),
    builder:
        (context, child) => Theme(
          data: ThemeData.light().copyWith(
            textTheme: TextTheme(titleSmall: TextStyle(fontSize: 16)),
            datePickerTheme: DatePickerThemeData(
              dayStyle: TextStyle(fontSize: 12),
            ),
          ),
          child: child!,
        ),
  );

Now:

  showDatePicker(
    context: context,
    firstDate: DateTime(2025),
    lastDate: DateTime(2026),
    builder:
        (context, child) => DatePickerTheme(
          data: DatePickerThemeData(
            toggleModeStyle: TextStyle(fontSize: 16),
            dayStyle: TextStyle(fontSize: 12),
          ),
          child: child!,
        ),
  );

Ref #163866
Ref #160591

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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