change "enableCustomDesigns" to true in the withoutawallpaper-server-config file
add your custom design to "customDesigns" in the same file
For example, if you want to add a design named "test" with two colors, where the first can be any color and the second can be blue or green, you need to add "test[all;blue,green]".
each design must have an individual name
each design can have one to three colors
if you want you can send me your design on my discord (see below) that I can implement them in the mod

resources must be located in "<game folder>/config/withoutawallpaper/CustomWallpaperResources/" or in a resource pack
for each design you have to create a folder with the name of the design in "assets/withoutawallpaper/textures/block/wallpaper/"
create a "design.png" file and a "color0" folder, for more than one color additionally a "color1" folder and for more than two colors additionally a "color2" folder (the design.png image will be shown in the pasting table design selection)
in each "color0" / "color1" / "color2" folder you have to create a "<colorname>.png" file for each available color (for example "blue.png" or "light_gray.png")
if you have a wallpaper with lime as first color and black as second color, color0/lime.png and color1/black.png will be rendered on top of each other

Let me know:

WithoutAWallpaper