import 'package:flutter/material.dart';

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  bool? value = false;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Checkbox(
          value: value,
          side: MaterialStateBorderSide.resolveWith((Set<MaterialState> states) {
            if (states.contains(MaterialState.selected)) {
              return BorderSide(width: 2, color: Colors.green);
            }
            return null; // use the default for other states
          }),
          onChanged: (bool? newValue) {
            setState(() { value = newValue; });
          },
        ),
      ),
    );
  }
}

void main() {
  runApp(MaterialApp(home: Home()));
}