At one point the Chat Overlay did have an Opacity Slider., and this option is still editable via the Windows Registry... but for now the best way to add Chat to the (ReLive) Streaming is to instead use the Browser Element.
This does still have the Transparency Option.
Obviously the issue is that while for Twitch the URL doesn't change... for YouTube, it is a unique address each Stream.
So keep that in mind.
Ideally, what would be good is not just to have the Opacity Slider restored to Chat, but also see the addition of a Chroma Key Scene Elements (including Chat).