Thanks for checking out our game! On the background:
The shader is actually pretty simple, the gdshader code for that is here on our github repo. Pretty much just 2 copies of the same texture, each at 50% transparency, one is just scrolling up and left slowly, the other is scrolling down and right and being horizontally distorted based on a sine function.
To make the texture itself, i used Paint.NET. The lines you see in the texture are from this plugin that randomly generates mazes. Using a black-and-white maze as the base image, I rendered random red-orangish noise in a separate layer and drew some squares on a third layer using blue/purples. I set both those additional layers to XOR color blending which is what makes the "color splatter" type effect you see in the image. After that I used a filter to reduce the image to 4-colors, and recolored it based on our game's pallete. Then the final step was making the texture repeat seamlessly; theres a lot of ways you can accomplish that but I just ended up using another paint.NET plugin, a side effect of that is that you can see hard cutoffs where the red/blue changes to brown in the final image (though I ended up liking that look).
Link to the random maze plugin I used: https://forums.paint.net/topic/8448-random-mazes-ymd100725/
Link to the seamless texture maker plugin I used: https://forums.paint.net/topic/4591-seamless-texture-maker-ymd100718/