https://dartpad.dev/?id=63c396c3695c99c0457ffddf6df62feb

wallpaper.dart

import 'package:flutter/material.dart';

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

double width = 400;
double height = 400;
double thick = 4;
double side = 109;

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('Wallpaper')),
        body: Center(
          child: CustomPaint(
            painter: MyCustomPainter(),
            child: Container(
              width: width,
              height: height,
            ),
          ),
        ),
      ),
    );
  }
}

class MyCustomPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final paint = Paint()
      ..color = Colors.black
      ..strokeWidth = 1.0;

    for (double i = 1; i <= 100; i++) {
      for (double j = 1; j <= 100; j++) {
        double x = i * side / 100;
        double y = j * side / 100;
        int c = (x * x + y * y).toInt();
        if (c % 2 == 0) {
          plot(i, j, thick, paint, canvas);
        }
      }
    }
  }

  void plot(double x, double y, double thick, Paint p, Canvas c) {
    c.drawRect(
        Rect.fromPoints(
          Offset(x * thick, y * thick),
          Offset((x + 1) * thick, (y + 1) * thick),
        ),
        p);
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
    return false;
  }
}