Ha.. thats a very cool way of adjusting. themes. I hoped for being able to adjust a lot of things around markdown edit and markdown preview though. My biggest problem with Zed right now. Themes are actually ok to me. Maybe thats on their todo though who knows.
For markdown preview, a recent release added these two settings (top-level in your settings.json):
https://github.com/zed-industries/zed/pull/54003