HN has terrifically simply markup, your user agent should be able to display it ...

HN has terrifically simply markup, your user agent should be able to display it however you would like with relatively little configuration. What user agent are you using? All the popular ones that I'm familiar with (Firefox, Chrome derivatives) have addons or plugins for changing the appearance of websites. In Firefox you can even add rules to userContent.css without an addon.

Just for an example, here is my userContent.css for highlighting the five languages that I use most frequently in Google Translate.

  @-moz-document domain(translate.google.com) {
      [data-language-code="ar"],[data-language-code="en"],[data-language-code="el"],[data-language-code="iw"],[data-language-code="ru"] {
          background-color:#ddffdd !important;
      }
  }