// Wrap it in a Theme() Widget like this
Theme(
    data: Theme.of(context).copyWith(canvasColor: Colors.green),
    child: DropdownButton(
                  value: yourValue,
                  icon: Icon(Icons.keyboard_arrow_down, color: Colors.white),
                  onChanged: (value) {
                    
                  },
                  items: yourItems,
                  style: TextStyle(color: Colors.white),
         ),
),