Hey kurochan,
First of all thanks a lot for the support, I really appreciate it and I'm glad you like the tracks! Especially that you like specifically Kanagawa.wav and RGB Dash, those were actually my favourites to make in this pack :)
Feel free to post some links for your game, I'd love to check it out!
As for the background pattern: I actually made it through using a pattern I found in Canva, and then transferring it to Affinity/PS/Photopea (I believe all work the same with this), messing with the blending layers and using layer FX. I don't have a video sadly, as I usually just tweak things until the outcome looks nice.
But, if you end up struggling, feel free to send an email or DM on twitter, maybe I can show a screenshot of how I made it or something. Goodluck! :D