title: Flutter colorize background ExpansionPanelList
body: <p>in this below flutter <code>ExpansionPanelList</code> widget i'm trying to colorize white background to other color such as <code>indigo</code>, i can't find any documentation or helpful link to achieve that</p>
  
  <pre><code>ExpansionPanelList(
    expansionCallback: (int index, bool isExpanded) {
      setState(() {
        _categoryExpansionStateMap[_categoryExpansionStateMap.keys
            .toList()[index]] = !isExpanded;
      });
    },
    children: &lt;ExpansionPanel&gt;[
      ExpansionPanel(
          headerBuilder: (BuildContext context, bool isExpanded) {
            return ListTile(
              contentPadding: EdgeInsets.all(3.0),
              leading: Container(
                padding:
                    EdgeInsets.fromLTRB(10.0, 0.0, 10.0, 0.0),
                decoration: BoxDecoration(
                    border: Border(
                        left: BorderSide(
                            width: 3.0, color: Colors.white54))),
                child: Icon(
                  Icons.account_circle,
                  size: 30.0,
                  color: Colors.black,
                ),
              ),
              title: ListTile(
                contentPadding: EdgeInsets.all(0.0),
                title: Text(
                  Strings.yourBasicInformation,
                  style: TextStyle(
                      color: Colors.black,
                      fontSize: 14.0,),
                )
              ),
            );
          },
          body: Biography(),
          isExpanded: _categoryExpansionStateMap["Biography"]),
    ],
  ),
  </code></pre>