How can I decorate tab bar?

I’m new to Flutter. I want to make my own tab bar, as you can see in the picture.
I tried to do it with BoxDecoration, but I don’t know how and where get a decoration. I tried bottom: TabBar(…) and I tried in:

 child: DefaultTabController(length: 3,              
              child: Scaffold(backgroundColor: Colors.transparent,
                appBar: AppBar(

but don’t work. Help me please.