title: How to add background color for ExpansionPanel in Flutter
body: <p>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.</p>
  
  <p><strong>Card</strong> </p>
  
  <pre><code>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(),
    ),
  );
  </code></pre>
  
  <p><strong>Container</strong></p>
  
  <pre><code>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(),
    ),
  );
  </code></pre>