Not OP, but I use DarkReader and Noir and it doesn't remove/hide the upvote button for me on either. I do have color filters enabled in accessibility on iOS, though.
Just to document my experience with the DarkReader solution in this thread:
After using DarkReader for a day, I find it to be an unsatisfying solution. Not only does it mangle the HN UI, but worse, it shows the light interface for 5 to 10 seconds before switching to dark. Every time. This just partially solve reading HN at night.