Have you considered a user.js script? I'm using one to fill in the white borders and wrap comments at a reasonable width:

  // ==UserScript==
  // @match https://news.ycombinator.com/*
  // ==/UserScript==
  (function () {
    document.body.style.backgroundColor = "#f6f6ef";
    var s = document.createElement('style');
    s.innerText = "td.default {max-width: 40em; }";
    document.head.appendChild(s);
  })()

(For Chrome, name the file hn.user.js and drag/drop it onto the chrome://extensions page.)