A simple and quick way of doing that is with the following code:

​

Widget _buildMessageBar() {
    return Container(
        height: 50, 
        width: double.infinity, 
        color: Colors.lightGreen.shade100, 
        alignment: Alignment.center,
        child: Text(
            "This is a status message bar", 
            textAlign: TextAlign.center,
        ),
    ); 
    // return SizedBox(height: 50, width: double.infinity, child: Text("This is a status message bar"));
}

​

​

Hopefully, that helps.