Fix regression with ModalBottomSheets not responding to changes in theme (#42189)

Fix regression with ModalBottomSheets not responding to changes in theme
after they are shown.

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

@@ -380,6 +380,7 @@ class _ModalBottomSheetRoute<T> extends PopupRoute<T> {
 
   @override
   Widget buildPage(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+    final BottomSheetThemeData sheetTheme = theme?.bottomSheetTheme ?? Theme.of(context).bottomSheetTheme;
     // By definition, the bottom sheet is aligned to the bottom of the page
     // and isn't exposed to the top padding of the MediaQuery.
     Widget bottomSheet = MediaQuery.removePadding(
@@ -387,8 +388,8 @@ class _ModalBottomSheetRoute<T> extends PopupRoute<T> {
       removeTop: true,
       child: _ModalBottomSheet<T>(
         route: this,
-        backgroundColor: backgroundColor,
-        elevation: elevation,
+        backgroundColor: backgroundColor ?? sheetTheme?.modalBackgroundColor ?? sheetTheme?.backgroundColor,
+        elevation: elevation ?? sheetTheme?.modalElevation ?? sheetTheme?.elevation,
         shape: shape,
         clipBehavior: clipBehavior,
         isScrollControlled: isScrollControlled,
@@ -460,15 +461,13 @@ Future<T> showModalBottomSheet<T>({
   assert(debugCheckHasMediaQuery(context));
   assert(debugCheckHasMaterialLocalizations(context));
 
-  final BottomSheetThemeData theme = Theme.of(context).bottomSheetTheme;
-
   return Navigator.of(context, rootNavigator: useRootNavigator).push(_ModalBottomSheetRoute<T>(
     builder: builder,
     theme: Theme.of(context, shadowThemeOnly: true),
     isScrollControlled: isScrollControlled,
     barrierLabel: MaterialLocalizations.of(context).modalBarrierDismissLabel,
-    backgroundColor: backgroundColor ?? theme?.modalBackgroundColor ?? theme?.backgroundColor,
-    elevation: elevation ?? theme?.modalElevation ?? theme?.elevation,
+    backgroundColor: backgroundColor,
+    elevation: elevation,
     shape: shape,
     clipBehavior: clipBehavior,
   ));

packages/flutter/test/material/bottom_sheet_theme_test.dart

@@ -4,6 +4,7 @@
 
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
@@ -214,6 +215,75 @@ void main() {
     expect(material.elevation, 0);
     expect(material.color, null);
   });
+
+  testWidgets('Modal bottom sheets respond to theme changes', (WidgetTester tester) async {
+    const double lightElevation = 5.0;
+    const double darkElevation = 3.0;
+    const Color lightBackgroundColor = Colors.green;
+    const Color darkBackgroundColor = Colors.grey;
+
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData.light().copyWith(
+        bottomSheetTheme: const BottomSheetThemeData(
+          elevation: lightElevation,
+          backgroundColor: lightBackgroundColor,
+        ),
+      ),
+      darkTheme: ThemeData.dark().copyWith(
+        bottomSheetTheme: const BottomSheetThemeData(
+          elevation: darkElevation,
+          backgroundColor: darkBackgroundColor,
+        ),
+      ),
+      home: Scaffold(
+        body: Builder(
+          builder: (BuildContext context) {
+            return Column(
+              children: <Widget>[
+                RawMaterialButton(
+                  child: const Text('Show Modal'),
+                  onPressed: () {
+                    showModalBottomSheet<void>(
+                      context: context,
+                      builder: (BuildContext context) {
+                        return Container(
+                          child: const Text('This is a modal bottom sheet.'),
+                        );
+                      },
+                    );
+                  },
+                ),
+              ],
+            );
+          },
+        ),
+      ),
+    ));
+    await tester.tap(find.text('Show Modal'));
+    await tester.pumpAndSettle();
+
+    final Material lightMaterial = tester.widget<Material>(
+      find.descendant(
+        of: find.byType(BottomSheet),
+        matching: find.byType(Material),
+      ),
+    );
+    expect(lightMaterial.elevation, lightElevation);
+    expect(lightMaterial.color, lightBackgroundColor);
+
+    // Simulate the user changing to dark theme
+    tester.binding.window.platformBrightnessTestValue = Brightness.dark;
+    await tester.pumpAndSettle();
+
+    final Material darkMaterial = tester.widget<Material>(
+    find.descendant(
+      of: find.byType(BottomSheet),
+        matching: find.byType(Material),
+      ),
+    );
+    expect(darkMaterial.elevation, darkElevation);
+    expect(darkMaterial.color, darkBackgroundColor);
+  });
 }
 
 Widget bottomSheetWithElevations(BottomSheetThemeData bottomSheetTheme) {