So basically how it works is in the flie location "Custom UI/textures/ui/BACKGROUND/MENU_BG.png" basically it uses the image in that folder as the background

Warning: Other UI texture packs probably won’t work properly (Have tested with DéesseUI)

• .So I have this image in my BACKGROUND folder
• If you wanna replace the image go in the resource packs folder then find my resource pack then go to "/textures/ui/BACKGROUND" and replaced the image with the same name "MENU_BG.png"
• pretty simple
• ✅You can use the code in any of your resource packs 😁
• ✅public domain (license)
• ✅ you can publicly modify it 😀三
• ✅you can edit it without giving credit😀三
• ✅ A template for static backgrounds (I don't know how to make animated) to help other people😄三
• ✅ you can distribute it 
•  

😀Here is it in game. So yea its that simple.