If you want to change the background color of Popup MenuButton bullet window? That background color is based on the Theme , so you can change the color wrapping your PopMenuButton inside Theme and change the cardColor.
child: new PopupMenuButton(
If you like this question & answer and want to contribute, then write your question & answer and email to freewebmentor[@]gmail.com. Your question and answer will appear on FreeWebMentor.com and help other developers.