Border customisation

main.dart

import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.light().copyWith(scaffoldBackgroundColor: Colors.white),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          
          child: Padding(
            padding: EdgeInsets.all(20),
            child: Form(
              child: Column(
            children: [
              
              Field(),
              Field(),
            ],
          ))),
        ),
      ),
    );
  }
}

final focusedBorder = UnderlineInputBorder(
          borderSide: BorderSide(width: 2, color: Colors.grey[600]),
        );
final normalBorder = UnderlineInputBorder(
borderSide: BorderSide(width:0.3, color: Colors.grey[200]),
);

class Field extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return TextFormField(
      initialValue: "Blah",
      decoration: InputDecoration(
        focusedBorder: focusedBorder,
        border: normalBorder,
        
      ),
    );
  }
}