I got it working, but not styling MD in Plasmic via ClassName prop.
Recently did somethint similar. ClassName prop styles the container, but not items within. Since the output comes from the code component, you can‘t control the styling with the sidebar (or at least I don‘t know how). What I did instead was adding a class to the container (.markdown) and then used „Embedd CSS“ to target the markdown elements like „.markdown p“, „.markdown li“ etc. Worked for me