Making the window background a tiling image?

I’ve been working on a custom theme for Discord, and I’ve run into a problem. I designed this look for the windows, but I can’t figure out how to add the background. Most I’ve been able to do is have a stretched version around the edges, but that’s it. Is there any way to make it tile in the background? (in the image the first one is the default, the 2nd is the mock-up I made, and the 3rd is what I’ve been able to make so far)