[CupertinoActionSheet] Match colors to native (flutter#149568)

This PR matches the various colors of `CupertinoActionSheet` more closely with the native one. 

The following colors are changed.
* Sheet background color
* Pressed button color
* Cancel button color
* Pressed cancel button color
* Divider color
* Content text color

The resulting colors match with native one with deviation of at most 1 (in terms of 0~255 RGB).

The following are comparison (left to right: Native, Flutter after PR, Flutter current)
<img width="1295" alt="image" src="https://github.com/flutter/flutter/assets/1596656/3703a4a8-a856-42b1-9395-a6e14b1881ca">
<img width="1268" alt="image" src="https://github.com/flutter/flutter/assets/1596656/1eb9964e-41f1-414a-99ae-0a2e7da8d3fd">
_Note: The divider thickness is adjusted to `1/dpr` instead of 0.3 in both Flutter version to make them look more native, as will be proposed in flutter#149636

### Derivation 
All the colors are derived through color picker and calculation. The algorithm is as followed:
* Assume all colors are translucent grey colors, i.e. having the same value `x` for R, G, and B, with an alpha `a`.
* Given the barrier color is `x_B1=0` when the background is black, and `x_B2=204` when the background is white.
* Pick the target color `x_t1` when the background is black, and `x_t2` when the background is white
* Solve the following equations for `x` and `a`
```
a * x + (1-a) * x_B1 = x_t1
a * x + (1-a) * x_B2 = x_t2

a = 1 - (x_t1 - x_t2) / (x_B1 - x_B2)
x = (x_t1 - (1-a) * x_B1) / a
```

These equations use a linear model for color composition, which might not be exact, but is close enough for an accuracy of (1/255).

The full table is as follows:

<img width="1091" alt="image" src="https://github.com/flutter/flutter/assets/1596656/0fb76291-c3cc-4bb5-aefa-03ac6ac9bf1f">

* The first two columns are colors picked from XCode.
* The 3~4 columns are the colors picked from the current Flutter. Notice the deviation, which is sometimes drastic.
* The 5~6 columns are the colors picked from Flutter after this PR. The deviation is at most 1.
* The last few columns are calculation.
  * There are two rows whose calculation is based on adjusted numbers, since the original results are not accurate enough, possibly due to the linear composition.

During the calculation, I assumed these colors vary between light and dark modes, but it turns out that both modes use the same set of colors.

### Screenshots

Author: dkwingsmt

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

@@ -104,35 +104,35 @@ const Color _kDialogColor = CupertinoDynamicColor.withBrightness(
 // Translucent light gray that is painted on top of the blurred backdrop as the
 // background color of a pressed button.
 // Eyeballed from iOS 13 beta simulator.
-const Color _kPressedColor = CupertinoDynamicColor.withBrightness(
+const Color _kDialogPressedColor = CupertinoDynamicColor.withBrightness(
   color: Color(0xFFE1E1E1),
   darkColor: Color(0xFF2E2E2E),
 );
 
-const Color _kActionSheetCancelPressedColor = CupertinoDynamicColor.withBrightness(
-  color: Color(0xFFECECEC),
-  darkColor: Color(0xFF49494B),
-);
+// Translucent light gray that is painted on top of the blurred backdrop as the
+// background color of a pressed button.
+// Eyeballed from iOS 17 simulator.
+const Color _kActionSheetPressedColor = Color(0xCAE0E0E0);
+
+const Color _kActionSheetCancelColor = Color(0xFFFFFFFF);
+const Color _kActionSheetCancelPressedColor = Color(0xFFECECEC);
 
 // Translucent, very light gray that is painted on top of the blurred backdrop
 // as the action sheet's background color.
 // TODO(LongCatIsLooong): https://github.com/flutter/flutter/issues/39272. Use
 // System Materials once we have them.
-// Extracted from https://developer.apple.com/design/resources/.
-const Color _kActionSheetBackgroundColor = CupertinoDynamicColor.withBrightness(
-  color: Color(0xC7F9F9F9),
-  darkColor: Color(0xC7252525),
-);
+// Eyeballed from iOS 17 simulator.
+const Color _kActionSheetBackgroundColor = Color(0xC8FCFCFC);
 
 // The gray color used for text that appears in the title area.
-// Extracted from https://developer.apple.com/design/resources/.
-const Color _kActionSheetContentTextColor = Color(0xFF8F8F8F);
+// Eyeballed from iOS 17 simulator.
+const Color _kActionSheetContentTextColor = Color(0x851D1D1D);
 
 // Translucent gray that is painted on top of the blurred backdrop in the gap
 // areas between the content section and actions section, as well as between
 // buttons.
-// Eye-balled from iOS 13 beta simulator.
-const Color _kActionSheetButtonDividerColor = _kActionSheetContentTextColor;
+// Eyeballed from iOS 17 simulator.
+const Color _kActionSheetButtonDividerColor = Color(0xD4C9C9C9);
 
 // The alert dialog layout policy changes depending on whether the user is using
 // a "regular" font size vs a "large" font size. This is a spectrum. There are
@@ -1115,19 +1115,19 @@ class _ActionSheetButtonBackgroundState extends State<_ActionSheetButtonBackgrou
     BorderRadius? borderRadius;
     if (!widget.isCancel) {
       backgroundColor = isBeingPressed
-        ? _kPressedColor
-        : CupertinoDynamicColor.resolve(_kActionSheetBackgroundColor, context);
+        ? _kActionSheetPressedColor
+        : _kActionSheetBackgroundColor;
     } else {
       backgroundColor = isBeingPressed
-          ? _kActionSheetCancelPressedColor
-        : CupertinoColors.secondarySystemGroupedBackground;
+        ? _kActionSheetCancelPressedColor
+        : _kActionSheetCancelColor;
       borderRadius = const BorderRadius.all(Radius.circular(_kCornerRadius));
     }
     return MetaData(
       metaData: this,
       child: Container(
         decoration: BoxDecoration(
-          color: backgroundColor,
+          color: CupertinoDynamicColor.resolve(backgroundColor, context),
           borderRadius: borderRadius,
         ),
         child: widget.child,
@@ -2269,7 +2269,7 @@ class _CupertinoDialogActionsRenderWidget extends MultiChildRenderObjectWidget {
               : _kCupertinoDialogWidth,
       dividerThickness: _dividerThickness,
       dialogColor: CupertinoDynamicColor.resolve(_kDialogColor, context),
-      dialogPressedColor: CupertinoDynamicColor.resolve(_kPressedColor, context),
+      dialogPressedColor: CupertinoDynamicColor.resolve(_kDialogPressedColor, context),
       dividerColor: CupertinoDynamicColor.resolve(CupertinoColors.separator, context),
       hasCancelButton: _hasCancelButton,
     );
@@ -2283,7 +2283,7 @@ class _CupertinoDialogActionsRenderWidget extends MultiChildRenderObjectWidget {
             : _kCupertinoDialogWidth
       ..dividerThickness = _dividerThickness
       ..dialogColor = CupertinoDynamicColor.resolve(_kDialogColor, context)
-      ..dialogPressedColor = CupertinoDynamicColor.resolve(_kPressedColor, context)
+      ..dialogPressedColor = CupertinoDynamicColor.resolve(_kDialogPressedColor, context)
       ..dividerColor = CupertinoDynamicColor.resolve(CupertinoColors.separator, context)
       ..hasCancelButton = _hasCancelButton;
   }

packages/flutter/test/cupertino/action_sheet_test.dart

@@ -18,6 +18,69 @@ import 'package:flutter_test/flutter_test.dart';
 import '../widgets/semantics_tester.dart';
 
 void main() {
+  testWidgets('Overall looks correctly under light theme', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      TestScaffoldApp(
+        theme: const CupertinoThemeData(brightness: Brightness.light),
+        actionSheet: CupertinoActionSheet(
+          message: const Text('The title'),
+          actions: <Widget>[
+            CupertinoActionSheetAction(child: const Text('One'), onPressed: () {}),
+            CupertinoActionSheetAction(child: const Text('Two'), onPressed: () {}),
+          ],
+          cancelButton: CupertinoActionSheetAction(child: const Text('Cancel'), onPressed: () {}),
+        ),
+      ),
+    );
+
+    await tester.tap(find.text('Go'));
+    await tester.pumpAndSettle();
+
+    final TestGesture gesture = await tester.startGesture(tester.getCenter(find.text('One')));
+    // This golden file also verifies the structure of an action sheet that
+    // has a message, no title, and no overscroll for any sections (in contrast
+    // to cupertinoActionSheet.dark-theme.png).
+    await expectLater(
+      find.byType(CupertinoApp),
+      matchesGoldenFile('cupertinoActionSheet.overall-light-theme.png'),
+    );
+
+    await gesture.up();
+  });
+
+  testWidgets('Overall looks correctly under dark theme', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      TestScaffoldApp(
+        theme: const CupertinoThemeData(brightness: Brightness.dark),
+        actionSheet: CupertinoActionSheet(
+          title: const Text('The title'),
+          message: const Text('The message'),
+          actions: List<Widget>.generate(20, (int i) =>
+            CupertinoActionSheetAction(
+              onPressed: () {},
+              child: Text('Button $i'),
+            ),
+          ),
+          cancelButton: CupertinoActionSheetAction(child: const Text('Cancel'), onPressed: () {}),
+        ),
+      ),
+    );
+
+    await tester.tap(find.text('Go'));
+    await tester.pumpAndSettle();
+
+    final TestGesture gesture = await tester.startGesture(tester.getCenter(find.text('Button 0')));
+    // This golden file also verifies the structure of an action sheet that
+    // has both a message and a title, and an overscrolled action section (in
+    // contrast to cupertinoActionSheet.light-theme.png).
+    await expectLater(
+      find.byType(CupertinoApp),
+      matchesGoldenFile('cupertinoActionSheet.overall-dark-theme.png'),
+    );
+
+    await gesture.up();
+  });
+
   testWidgets('Verify that a tap on modal barrier dismisses an action sheet', (WidgetTester tester) async {
     await tester.pumpWidget(
       createAppWithButtonThatLaunchesActionSheet(
@@ -1675,6 +1738,50 @@ Widget createAppWithButtonThatLaunchesActionSheet(Widget actionSheet) {
   );
 }
 
+// Shows an app that has a button with text "Go", and clicking this button
+// displays the `actionSheet` and hides the button.
+//
+// The `theme` will be applied to the app and determines the background.
+class TestScaffoldApp extends StatefulWidget {
+  const TestScaffoldApp({super.key, required this.theme, required this.actionSheet});
+  final CupertinoThemeData theme;
+  final Widget actionSheet;
+
+  @override
+  TestScaffoldAppState createState() => TestScaffoldAppState();
+}
+
+class TestScaffoldAppState extends State<TestScaffoldApp> {
+  bool _pressedButton = false;
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoApp(
+      theme: widget.theme,
+      home: Builder(builder: (BuildContext context) =>
+        CupertinoPageScaffold(
+          child: Center(
+            child: _pressedButton ? Container() : CupertinoButton(
+              onPressed: () {
+                setState(() {
+                  _pressedButton = true;
+                });
+                showCupertinoModalPopup<void>(
+                  context: context,
+                  builder: (BuildContext context) {
+                    return widget.actionSheet;
+                  },
+                );
+              },
+              child: const Text('Go'),
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}
+
 Widget boilerplate(Widget child) {
   return Directionality(
     textDirection: TextDirection.ltr,