CustomPainter for diagonal stripes. PS: Check the first comment

stripes_painter.dart

import 'package:flutter/material.dart';

class StripesPainter extends CustomPainter {
  StripesPainter({required this.stripeColor});

  final Color stripeColor;
  @override
  void paint(Canvas canvas, Size size) {
    final paint = Paint()
      ..color = stripeColor
      ..strokeWidth = 10;
    var stepX = .0;
    var stepY = size.height;

    const step = 10;

    while (stepX < size.width * 2) {
      canvas.drawLine(
        Offset(-4, stepY -= step),
        Offset(stepX += step, size.height + 4),
        paint,
      );

      stepY -= step * 2;
      stepX += step * 2;
    }
  }

  @override
  bool shouldRepaint(StripesPainter oldDelegate) => false;
}

/// Use ClipRect to prevent stripes from drawing out of parents widget bounds
ClipRect(
    child: CustomPaint(
        painter: StripesPainter(
            stripeColor: Colors.red,  // Choose stripe color
        )
    )
)