remove default backbutton background color

I would like to remove the background color of the default backbutton. I have below configurations but it still appears like in the photo. TIA for all inputs.

hightlightColor: Colors.transparent,
splashColor: Colors.transparent,
splashFactory: NoSplash.splashFactory,

Thanks in advance.

I found a way to override the theming in the whole app. Set the following in ThemeData and provide it to theme in MaterialApp.

iconButtonTheme: IconButtonThemeData(
        style: ButtonStyle(
          overlayColor: MaterialStateProperty.all(Colors.transparent),
          splashFactory: NoSplash.splashFactory,
        ),
      ),

Additionally, to remove all tooltip in the automaticallyImplyLeading button, I came across the following:

class BackButtonOverride extends DefaultMaterialLocalizations {
  BackButtonOverride(Locale locale) : super();

  @override
  String get backButtonTooltip => '';
}

class BackButtonDelegate
    extends LocalizationsDelegate<MaterialLocalizations> {
  const BackButtonDelegate();
  @override
  Future<BackButtonOverride> load(Locale locale) {
    return SynchronousFuture(BackButtonOverride(locale));
  }

  @override
  bool shouldReload(BackButtonDelegate old) => false;
  @override
  bool isSupported(Locale locale) => true;
}

and applied to MaterialApp as follows:

localizationsDelegates: const [
              BackButtonDelegate(),
            ],