If you set nothing then you get unexpected behavior, as has occurred. My advice is to set background color and not rely on Twitch doing what you expect it to, bugs or no bugs in the rig’s behavior.
Yes it’s probably a rig bug, but don’t rely on Twitch always being white background, and you using transparent/no set CSS.
See also:
If you use a transparent background, and you get this bug, none of your text is going to be readable, because you didn’t set a none transparent background.