This is part of a Live Mapping session demonstrating the Ryecroft Town annual issue, but the Street tool works the same in any style.

https://youtu.be/cIwOSDF9VPw?t=1642

This is unofficially known as Transparency Acne, and is an artefact caused by the rendering engine when it fails to recognise pixels in the upper sheet that happen to be exactly the same colour as the underlying pixels.

This causes little virtual holes (holes) in the upper sheet, and effects are applied around the edge of each of these holes. In this example the bevel is being applied, resulting in this pitted effect.

Add a new sheet just underneath the upper sheet and copy the the polygons from the upper sheet onto it, then hide everything except the new sheet and use the Change Properties tool to make the new polygons on the underlying sheet a solid fill and some colour that you can be fairly sure isn't included in the texture. Colour 227 is a good one.

This issue of the 2021 Cartographer's Annual has just been published, so here is the full development example map to round off the end of this thread.

Ok. I think that will probably be a bit too small to look good.

I've trimmed it to work as a symbol, but I'm not sure it will look all that great on a background that is different to the one it began its life on, but if it works it works, and if not, it doesn't.

Are you looking for a nebula symbol, or for a complete image to use as a background?

I have a relatively simple nebula image (not a real one, but one I made in GIMP). You could have this one, but I don't know if it is what you are looking for. It would have to be a background image only as it isn't a seamless texture.

If you do want it, let me know what size (it's square).