Adding a border to a widget is very easy in Flutter. We just need to wrap the widget in a Container and add BoxDecoration to it.
Let’s say we want to make a square with blue borders all we need to do is:
Container(
height: 100,
width: 100,
decoration: BoxDecoration(
border: Border.all(
color: Colors.blue,
),
borderRadius: BorderRadius.circular(10.0),
),
child: Center(
child: Text('mrflutter.com'),
),
),
Which will look like this:

What if the border is too “pointy” and we want a nice border-radius? No problem, we just add borderRadius to our BoxDecoration constructor:
decoration: BoxDecoration(
border: Border.all(
color: Colors.blue,
),
borderRadius: BorderRadius.circular(10.0),
),
We then get the following:

You can also choose on what sides of the container you want a border using the BorderSide constructor.
decoration: BoxDecoration(
border: Border(
top: BorderSide(
color: Colors.blue,
// width: 3.0 --> you can set a custom width too!
),
bottom: BorderSide(
color: Colors.blue,
),
),
),
Which looks like this:

I hope this post helps you utilize the widgets that Flutter offers to create a custom border to your own widgets.