How to theme rounded buttons with no drop shadow?

[doodatales]

Maybe I'm missing something, but I can't find a way to make the sign in, icon buttons, etc themed as rounded (circular) buttons. The rest of my app has rounded buttons so having rectangular rounded corner buttons for login doesn't make sense from a design perspective. For example, changing the theme for icon buttons doesn't affect the Facebook, Google, Apple icon buttons. I have the same issue removing the drop shadow.

    iconButtonTheme: IconButtonThemeData(
        style: ButtonStyle(
            shape: MaterialStateProperty.all(
              RoundedRectangleBorder(
                borderRadius: BorderRadius.circular(30),
              ),
            ),
            shadowColor: MaterialStateProperty.all(Colors.black),
            elevation: MaterialStateProperty.all(0))),