class MyWidget extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Theme(
data: ThemeData(primaryColor: Colors.purple),
child: TextField(
decoration: InputDecoration(
labelText: "Password",
fillColor: Colors.white,
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(30.0),
borderSide: BorderSide(
color: Colors.purple,
),
),
enabledBorder: OutlineInputBorder(
borderRadius: BorderRadius.circular(30.0),
borderSide: BorderSide(
color: Colors.purple,
width: 2.0,
),
),
),
),
);
}
}
Adding a gradient to the border seems pretty difficult. You can check this question for more info.