Flutter – Creating the Outlined button is very easy. The OutlinedButton widget allows you to create outlined buttons in an easy way. so, let’s get started.
Creating OutlinedButton
First, create a text widget and give the button name.
Text("Outlined Button")
Then, wrap the text widget as OulinedButton Widget.
OutlinedButton(
onPressed: () {},
child: Text("Outlined Button"),
),

To customize the OutlinedButton widget use style property. Define OulinedButton in style property. The examples are given below,
Changing Outline/border color
OutlinedButton(
style: OutlinedButton.styleFrom(
side: BorderSide(color: Colors.blue, width: 2),
),
onPressed: () {},
child: Text(
"Outlined Button",
style: TextStyle(color: Colors.black),
),
),

Changing Button background color
OutlinedButton(
style: OutlinedButton.styleFrom(
backgroundColor: Colors.blue[200],
),
onPressed: () {},
child: Text(
"Outlined Button",
style: TextStyle(color: Colors.black),
),
),

Changing Text Style
For more info about text style, visit Text Widget Post.
OutlinedButton(
child: Text("Outlined Button"),
style: OutlinedButton.styleFrom(
backgroundColor: Colors.blue,
primary: Colors.white,
textStyle: TextStyle(
color: Colors.black,
fontSize: 20,
fontStyle: FontStyle.italic,
),
),
onPressed: () {
},
),

Changing Shadow and Elevation
OutlinedButton(
child: Text("Outlined Button"),
style: OutlinedButton.styleFrom(
backgroundColor: Colors.blue,
primary: Colors.white,
shadowColor: Colors.red,
elevation: 10,
),
onPressed: () {},
),

Changing Shape
OutlinedButton(
child: Text("Outlined Button"),
style: OutlinedButton.styleFrom(
backgroundColor: Colors.blue,
primary: Colors.white,
shape: const RoundedRectangleBorder(
borderRadius: BorderRadius.all(
Radius.circular(18),
),
),
),
onPressed: () {},
),

Reference: visit flutter.dev