border_hairline.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.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: Column(
            children: <Widget>[
              Container(
                margin: EdgeInsets.all(16),
                height: 200,
                width: 200,
                decoration: BoxDecoration(
                  border: Border(
                    top: BorderSide(color: Colors.white, width: 0),
                    left: BorderSide(color: Colors.white, width: .1),
                    bottom: BorderSide(color: Colors.white, width: 1),
                  ),
                ),
              ),
              Container(
                margin: EdgeInsets.all(16),
                height: 200,
                width: 200,
                decoration: BoxDecoration(
                  border: Border.all(color: Colors.white, width: 0),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}