Fix DateRangePickerDialog does not use rangePickerHeaderBackgroundColor from DatePickerTheme in M2 (#147370)

fixes [DatePickerThemeData's `rangePickerHeaderBackgroundColor` not being applied to DateRangePickerDialog on M2](https://github.com/flutter/flutter/issues/147301)

### 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: false,
        datePickerTheme: const DatePickerThemeData(
          rangePickerHeaderBackgroundColor: Colors.red,
        ),
      ),
      home: Scaffold(
        body: Center(
          child: Builder(builder: (BuildContext context) {
            return ElevatedButton(
              onPressed: () {
                showDateRangePicker(
                  context: context,
                  currentDate: DateTime.now(),
                  initialDateRange: DateTimeRange(
                    start: DateTime.now(),
                    end: DateTime.now().add(const Duration(days: 7)),
                  ),
                  firstDate: DateTime(2000),
                  lastDate: DateTime(2025),
                );
              },
              child: const Text('Show Date Range Picker'),
            );
          }),
        ),
      ),
    );
  }
}
```

</details>

### When using `rangePickerHeaderBackgroundColor` in `DatePickerTheme` for M2

```dart
      theme: ThemeData(
        useMaterial3: false,
        datePickerTheme: const DatePickerThemeData(
          rangePickerHeaderBackgroundColor: Colors.red,
        ),
      ),
```

| Before | After |
| --------------- | --------------- |
| <img src="https://github.com/flutter/flutter/assets/48603081/4e85a115-c702-45b2-8f1e-2f3c87c37629" /> | <img src="https://github.com/flutter/flutter/assets/48603081/d002ba0a-6328-4f97-8ba0-dcbb9662bbb3"  /> |

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

@@ -1660,7 +1660,7 @@ class _CalendarRangePickerDialog extends StatelessWidget {
           actionsIconTheme: iconTheme,
           elevation: useMaterial3 ? 0 : null,
           scrolledUnderElevation: useMaterial3 ? 0 : null,
-          backgroundColor: useMaterial3 ? headerBackground : null,
+          backgroundColor: headerBackground,
           leading: CloseButton(
             onPressed: onCancel,
           ),

packages/flutter/test/material/date_picker_theme_test.dart

@@ -539,7 +539,7 @@ void main() {
     );
 
     final Material material = findDialogMaterial(tester);
-    expect(material.color, datePickerTheme.backgroundColor); //!!
+    expect(material.color, datePickerTheme.backgroundColor);
     expect(tester.widget<Scaffold>(find.byType(Scaffold)).backgroundColor, datePickerTheme.rangePickerBackgroundColor);
     expect(material.elevation, datePickerTheme.rangePickerElevation);
     expect(material.shadowColor, datePickerTheme.rangePickerShadowColor);
@@ -573,6 +573,67 @@ void main() {
     expect(inkFeatures, paints..circle(color: datePickerTheme.rangeSelectionOverlayColor?.resolve(<MaterialState>{})));
   });
 
+  testWidgets('Material2 - DateRangePickerDialog uses ThemeData datePicker theme', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(
+          datePickerTheme: datePickerTheme,
+          useMaterial3: false,
+        ),
+        home: Directionality(
+          textDirection: TextDirection.ltr,
+          child: Material(
+            child: Center(
+              child: DateRangePickerDialog(
+                firstDate: DateTime(2023),
+                lastDate: DateTime(2023, DateTime.january, 31),
+                initialDateRange: DateTimeRange(
+                  start: DateTime(2023, DateTime.january, 17),
+                  end: DateTime(2023, DateTime.january, 20),
+                ),
+                currentDate: DateTime(2023, DateTime.january, 23),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    final Material material = findDialogMaterial(tester);
+    expect(material.color, datePickerTheme.backgroundColor);
+    expect(tester.widget<Scaffold>(find.byType(Scaffold)).backgroundColor, datePickerTheme.rangePickerBackgroundColor);
+    expect(material.elevation, datePickerTheme.rangePickerElevation);
+    expect(material.shadowColor, datePickerTheme.rangePickerShadowColor);
+    expect(material.surfaceTintColor, datePickerTheme.rangePickerSurfaceTintColor);
+    expect(material.shape, datePickerTheme.rangePickerShape);
+
+    final AppBar appBar = tester.widget<AppBar>(find.byType(AppBar));
+    expect(appBar.backgroundColor, datePickerTheme.rangePickerHeaderBackgroundColor);
+
+    final Text selectRange = tester.widget<Text>(find.text('SELECT RANGE'));
+    expect(selectRange.style?.color, datePickerTheme.rangePickerHeaderForegroundColor);
+    expect(selectRange.style?.fontSize, datePickerTheme.rangePickerHeaderHelpStyle?.fontSize);
+
+    final Text selectedDate = tester.widget<Text>(find.text('Jan 17'));
+    expect(selectedDate.style?.color, datePickerTheme.rangePickerHeaderForegroundColor);
+    expect(selectedDate.style?.fontSize, datePickerTheme.rangePickerHeaderHeadlineStyle?.fontSize);
+
+    // Test the day overlay color.
+    final RenderObject inkFeatures = tester.allRenderObjects.firstWhere((RenderObject object) => object.runtimeType.toString() == '_RenderInkFeatures');
+    final TestGesture gesture = await tester.createGesture(
+      kind: PointerDeviceKind.mouse,
+    );
+    await gesture.addPointer();
+    await gesture.moveTo(tester.getCenter(find.text('16')));
+    await tester.pumpAndSettle();
+    expect(inkFeatures, paints..circle(color: datePickerTheme.dayOverlayColor?.resolve(<MaterialState>{})));
+
+    // Test the range selection overlay color.
+    await gesture.moveTo(tester.getCenter(find.text('18')));
+    await tester.pumpAndSettle();
+    expect(inkFeatures, paints..circle(color: datePickerTheme.rangeSelectionOverlayColor?.resolve(<MaterialState>{})));
+  });
+
   testWidgets('Dividers use DatePickerThemeData.dividerColor', (WidgetTester tester) async {
     Future<void> showPicker(WidgetTester tester, Size size) async {
       tester.view.physicalSize = size;

packages/flutter/test/material/date_range_picker_test.dart

@@ -1576,7 +1576,7 @@ void main() {
         expect(appBar.actionsIconTheme, iconTheme);
         expect(appBar.elevation, null);
         expect(appBar.scrolledUnderElevation, null);
-        expect(appBar.backgroundColor, null);
+        expect(appBar.backgroundColor, theme.colorScheme.primary);
       });
     });