UX is always built on the server side.
If you want to tweak it, use a browser extension. The project didn’t create Discourse, so as @hellcp stated in the ticket you opened, if you want DIscourse to have options to do this, you need to open a ticket in the upstream project. We’re not interested in maintaining a fork of Discourse because a small number of users want tweaks to the UI.
You’re free to do whatever you want with Stylus or Tampermonkey to format things just the way you want them. Then if an update to the software breaks it for you, you can work on fixing your custom CSS.