I’m deathly allergic to Helvetica and would love CSS control over margins, leading, etc. in the Drafts Markdown Preview window. Is this currently possible?
You can apply any CSS, in the HTML Preview action step. It can either be written directly in that step, or imported via a script step before it. (See the docs for it.)
Also, if you look at the D4 action directory, there are a number of previews available with CSS that you should be able to modify with what you need.