Yeah. I don't really understand why this feature even exists. Maybe it's some artifact of HN's history, pg experimenting with that particular part of HN's engine in Arc or something? Because this and all the other things you've mentioned can be accomplished by user setting up an additional CSS for the website client-side, with an implicit assumption that audience here is generally smart enough to figure out how to do this.

Also, the orange bar is basically the single visual element that makes HN HN, so I don't know why people would even like to customize it...

There's some mystery here.