bug: IconTheme is overridden when using CupertinoScaffold

[Cteq3132]

Hey,

Thanks for this package, it helps us a lot with modals & bottom sheets on our projects.

I noticed what I believe is a bug when using a CupertinoScaffold in my app. My app has a custom IconTheme, with a fixed size to 20, a fixed color, ... If I get the IconTheme from anywhere in my app with the following, everything is fine.

final iconTheme = IconTheme.of(context);
final size = iconTheme.size;
print(size); //Result is 20

When I add a CupertinoScaffold on top of it, then getting the icon theme size is not ok anymore:

final iconTheme = IconTheme.of(context);
final size = iconTheme.size;
print(size); //Result is 24

After a bit of investigation, I believe it's due to the _CupertinoModalTransition adding a CupertinoTheme, that adds an IconTheme in it's build method with a fixed CupertinoIconThemeData: flutter/lib/src/cupertino/theme.dart file, line 121 to 126:

@override
  Widget build(BuildContext context) {
    return _InheritedCupertinoTheme(
      theme: this,
      child: IconTheme(
        data: CupertinoIconThemeData(color: data.primaryColor),
        child: child,
      ),
    );
  }

To fix this temporarily, I am overriding the IconTheme under the CupertinoScaffold with my previous IconTheme but it's more a hack than a fix:

@override
Widget build(BuildContext context) {
  final iconTheme = IconTheme.of(context);
  
  return CupertinoScaffold(
    body: IconTheme(
      data: iconTheme,
      child: child,
    ),
  );
}

Any idea how to fix this properly? I would be glad to help if someone has an idea for a clean fix.