Background
Transparent, green screen or a colour of your own.
Transparent
The window has a real alpha channel. Add it in OBS as a window capture — no keying needed.
Warning: Transparency needs a compositor that supports it. On Linux that depends on the window manager.
Green screen
Classic green, for keying with the chroma key filter. The safe route when transparency does not make it through.
Custom colour
For cases where green appears in the avatar — then magenta or blue is better.
TODO: Recommended chroma key settings for OBS.