Do you mean like how HN puts a number next to your name?
I used to not be able to: I set my bar to #000000, which is the same color as the text. And unless I'm imagining things, somewhere along the way it changed to #222222
I can set the topbar to #222222, but then everything else shows up in an annoying low-contrast shadow
You can't, period. There used to be a comment-level counter years ago, but it was removed as part of an experiment to see if not having it improves overall discussion quality. Apparently the experiment concluded positively, since the counter hasn't been brought back.