Fix `CupertinoDatePicker`dark mode text color (#100312)

Improved code

packages/flutter/lib/src/cupertino/date_picker.dart

@@ -53,7 +53,9 @@ const double _kTimerPickerColumnIntrinsicWidth = 106;
 
 TextStyle _themeTextStyle(BuildContext context, { bool isValid = true }) {
   final TextStyle style = CupertinoTheme.of(context).textTheme.dateTimePickerTextStyle;
-  return isValid ? style : style.copyWith(color: CupertinoDynamicColor.resolve(CupertinoColors.inactiveGray, context));
+  return isValid
+    ? style.copyWith(color: CupertinoDynamicColor.maybeResolve(style.color, context))
+    : style.copyWith(color: CupertinoDynamicColor.resolve(CupertinoColors.inactiveGray, context));
 }
 
 void _animateColumnControllerToItem(FixedExtentScrollController controller, int targetItem) {

packages/flutter/test/cupertino/date_picker_test.dart

@@ -15,6 +15,7 @@
 import 'dart:ui';
 
 import 'package:flutter/cupertino.dart';
+import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -1515,6 +1516,33 @@ void main() {
     expect(lastSelectedItem, 1);
     handle.dispose();
   });
+
+  testWidgets('DatePicker adapts to MaterialApp dark mode', (WidgetTester tester) async {
+    Widget _buildDatePicker(Brightness brightness) {
+      return MaterialApp(
+        theme: ThemeData(brightness: brightness),
+        home: CupertinoDatePicker(
+          mode: CupertinoDatePickerMode.date,
+          onDateTimeChanged: (DateTime neData) {},
+          initialDateTime: DateTime(2018, 10, 10),
+        ),
+      );
+    }
+
+    // CupertinoDatePicker with light theme.
+    await tester.pumpWidget(_buildDatePicker(Brightness.light));
+    RenderParagraph paragraph = tester.renderObject(find.text('October').first);
+    expect(paragraph.text.style!.color, CupertinoColors.label);
+    // Text style should not return unresolved color.
+    expect(paragraph.text.style!.color.toString().contains('UNRESOLVED'), isFalse);
+
+    // CupertinoDatePicker with dark theme.
+    await tester.pumpWidget(_buildDatePicker(Brightness.dark));
+    paragraph = tester.renderObject(find.text('October').first);
+    expect(paragraph.text.style!.color, CupertinoColors.label);
+    // Text style should not return unresolved color.
+    expect(paragraph.text.style!.color.toString().contains('UNRESOLVED'), isFalse);
+  });
 }
 
 Widget _buildPicker({