Only accept either background color or decoration on SurveyKit.

[eldrig0]

Currently adding SurveyKit( backgroundColor: Colors.red, ...)
does only applies color to AppBar and not the entire page and need to have decoration param like
SurveyKit( backgroundColor: Colors.red, decoration: BoxDecoration(color: Colors.red) ... )
to apply change color on entire page

[AlexHCJP]

 if (state is PresentingSurveyState) {
          return Scaffold(
            backgroundColor: Colors.transparent,
            appBar: state.currentStep.showAppBar
                ? PreferredSize(
                    preferredSize: Size(
                      double.infinity,
                      70.0,
                    ),
                    child: widget.appBar != null
                        ? widget.appBar!.call(state.appBarConfiguration)
                        : SurveyAppBar(
                            appBarConfiguration: state.appBarConfiguration,
                          ),
                  )
                : null,

need remove

backgroundColor: Colors.transparent