Hey there, I'm here looking for info on the "transparent" setting and stumbled on your comment. Did you figure things out? If not, here's a few tips:
Choose one of the basic BG colors. It's fine, believe me.
On OBS (or your streaming program of choice) you need to add a new window or game source (Not sure why I can't get a game source from OBS in Linux. eh). Choose Veadotube Mini and add a chroma key filter on it. Adjust the similarity settings as you see fit depending on your avatar colors.
Make sure the avatar source is on top of the actual game source, so it's showing over your game.
You might need to shrink the avatar down on OBS, just select the source and you can use the highlighted contour to reduce size.