How to add background color for ExpansionPanel in Flutter

I am trying to style ExpansionPanel in Flutter but the color is not applying to the whole panel. I have tried both Container and Card widget, the color is not updating. Any Ideas? I want to add background color to cover the entire ExpansionPanel. Is there a way to add the parent theme to the ExpansionPanel.

Card

Card(
  elevation: 2.0,
  color: Theme.of(context).primaryColor,
  margin: EdgeInsets.only(left: 10.0,right: 10.0,top: 10.0),
  child: ExpansionPanelList(
    expansionCallback: (int index, bool isExpanded) {
      setState(() {
        _items[index].isExpanded = !_items[index].isExpanded;
        Timer(Duration(milliseconds: 200), () {
          setState(() {
            _reconfigureFAB();
          });
        });
      });
    },
    children: _items.map((IncludedItem item) {
      return ExpansionPanel(
        headerBuilder: (BuildContext context, bool isExpanded) {
          return Container(
            padding: EdgeInsets.only(left: 18.0),
            child: Row(children: [
              Text(
                "What's included",
                textAlign: TextAlign.start,
                style: TextStyle(
                    fontFamily: 'Bold',
                    fontSize: 33.0,
                    color: Theme.of(context).backgroundColor),
              ),
            ]),
          );
          ;
        },
        isExpanded: item.isExpanded,
        body: Container(
          child: Text("body"),
        ),
      );
    }).toList(),
  ),
);

Container

Container(
  color: Theme.of(context).primaryColor,
  margin: EdgeInsets.only(left: 10.0,right: 10.0,top: 10.0),
  child: ExpansionPanelList(
    expansionCallback: (int index, bool isExpanded) {
      setState(() {
        _items[index].isExpanded = !_items[index].isExpanded;
        Timer(Duration(milliseconds: 200), () {
          setState(() {
            _reconfigureFAB();
          });
        });
      });
    },
    children: _items.map((IncludedItem item) {
      return ExpansionPanel(
        headerBuilder: (BuildContext context, bool isExpanded) {
          return Container(
            padding: EdgeInsets.only(left: 18.0),
            child: Row(children: [
              Text(
                "What's included",
                textAlign: TextAlign.start,
                style: TextStyle(
                    fontFamily: 'Bold',
                    fontSize: 33.0,
                    color: Theme.of(context).backgroundColor),
              ),
            ]),
          );
          ;
        },
        isExpanded: item.isExpanded,
        body: Container(
          child: Text("body"),
        ),
      );
    }).toList(),
  ),
);

ExpansionPanelList uses cardColor color from your theme. You can specify it in MaterialApp (theme property) or override it in your widget:

Container(
          color: Theme.of(context).primaryColor,
          margin: EdgeInsets.only(left: 10.0, right: 10.0, top: 10.0),
          child: Theme(
            data: Theme.of(context).copyWith(cardColor: Colors.red),
            child: ExpansionPanelList(
                ...