In case you weren’t aware, there are extensions that you can use to restore the older (better) UIs. Here are a couple:

• https://addons.mozilla.org/en-US/firefox/addon/wikipedia-vector-skin/
• https://addons.mozilla.org/en-US/firefox/addon/classic-wikipedia/

There are probably some for other browsers as well. I don’t use them though. I instead wrote myself a tampermonkey script to change it:

if (!window.location.search.contains('useskin')) {
  var new_url = window.location.protocol
      + "//" + window.location.hostname
      + window.location.pathname;
  if (window.location.search == "") {
    new_url = new_url + "?useskin=monobook";
  } else {
    new_url = new_url + window.location.search + "&useskin=monobook";
  }
  new_url = new_url + window.location.hash;
  window.location.replace(new_url);
}

You can compare the available wikipedia styles on this page to see which one you like best: https://en.wikipedia.org/wiki/Wikipedia:Skin?useskin=monobook