i've never used lightstream, but it should have the option to enable transparency; if neither that nor chroma key work then there isn't much to do :[
I got with their chat support and figured a work around. It's kind of silly since the whole point of LightStream is you use it instead of OBS to stream from a Playstation or Xbox and they add your overlays. The workaround requires that I set up my background and PNGTuber in OBS and use a virtual OBS camera as a source in Lightstream. Then I add the Stream Elements overlays in Lightstream. It's a klunky workaround. It also seems to create a minor delay from veadotube. I'll keep on them to add Alpha Channel Support for browser windows. They don't support green screen for cams either yet, so those people have to do the OBS virtual camera too. Seems like they should make that a priority.