Sorry for the confusion. In the OP I was trying to communicate what files I tried making, and the locations in which I placed them. I don’t want to name the chat mode differently, I’m just trying to figure out which scss file I should place the css in to effect only the desktop, full screen mode chat.
You can put your CSS in theme/desktop/desktop.scss — to impact chat it doesn’t have to be in a chat-specific file, you just have to target the relevant classes.