在我的自定义表单字段中，我想获取应用程序的默认fillColor并在自定义控件中使用它。正确的方法是什么？以下方法返回null，即使默认TextFormField填充为灰色。

class CustomFormField extends FormField<String> {
  CustomFormField({
    String? initialText,
    FormFieldSetter<String>? onSaved,
    FormFieldValidator<String>? validator,
    AutovalidateMode? autovalidateMode,
    Key? key,
  }) : super(
          onSaved: onSaved,
          validator: validator,
          initialValue: initialText,
          autovalidateMode: autovalidateMode,
          key: key,
          builder: (FormFieldState<String> state) {
            var fillColor =
                Theme.of(state.context).inputDecorationTheme.fillColor;
            // ISSUE: fillColor is null, even though the default 
            // TextFormField fills with a shade of grey in the same app. 
            return Container(
                // with custom control UI using fillColor
                );
          },
        );
}