Simple example to convert your widget into the Mouse Cursor in Flutter Web

Flutter Web Custom Cursors

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

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

void main() {
  runApp(
    MaterialApp(
      theme: ThemeData(scaffoldBackgroundColor: darkBlue),
      home: MyWidget(),
    ),
  );
}

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  Offset pointer = Offset.zero;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: MouseRegion(
          cursor: SystemMouseCursors.none,
          onHover: (eve) {
            setState(() {
              pointer = eve.position;
            });
          },
          child: Stack(
            children: [
              AnimatedPositioned(
                duration: const Duration(milliseconds: 500),
                left: pointer.dx - 100,
                top: pointer.dy - 100,
                child: Container(
                  height: 200,
                  width: 200,
                  decoration: BoxDecoration(
                      borderRadius: BorderRadius.all(Radius.circular(100)),
                      border: Border.all(
                          width: 4,
                          color: Colors.white,
                          style: BorderStyle.solid)),
                ),
              ),
            ],
          )),
    );
  }
}