Bear in mind I’m an amateur artist. My skills with GIMP and Inkscape are rudimentary. So I decided to start easy by adapting other Linux wallpapers. I came from Manjaro and this was one of my favorites. Everything except the background hemisphere was produced from scratch in Inkscape.
I personally think it would be a neat feature if the Qubes logo itself was saved as a png with transparent background so you could pick the main background color yourself without limiting it to only the default gray
Edit: The point of doing it that way would obviously be to avoid moving random files from the internet into dom0, but upon further research it seems there are already a variety of different qubes logos in /usr/share/backgrounds