firefox/styles/hacker-news.css

@-moz-document domain("news.ycombinator.com") {
  body,
  [bgcolor="#f6f6ef"],
  [bgcolor="#ff6600"],
  textarea,
  form[action="comment"] input {
    background-color: hsl(218deg 14% 18%);
  }
  
  .votearrow {
    filter: contrast(0%) invert() brightness(125%);
    mix-blend-mode: soft-light;
  }
  
  img[src="y18.gif"] {
    filter: grayscale(1) brightness(60%);
    mix-blend-mode: soft-light;
    margin-right: 8px;
  }

  form[action="comment"] input,
  form[action="comment"] textarea {
    border-color: currentColor;
    outline: none;
  }
  
  [action="//hn.algolia.com/"],
  .pagetop .hnname {
    display: none;
  }
  
  .subtext, .subtext a:link, .subtext a:visited,
  .comhead a:link,
  .rank,
  form[action="comment"] input {
    color: hsl(218deg 14% 33%);
  }
  
  a:link, a:visited,
  .pagetop a:visited,
  .toptext,
  form[action="comment"] textarea {
    color: hsl(218deg 14% 43%);
  }
  
  .c00, .c00 a:link, .c00 a:visited { color: hsl(218deg 14% 56%); }
  .c5a, .c5a a:link, .c5a a:visited { color: hsl(218deg 14% 43%); }
  .c73, .c73 a:link, .c73 a:visited { color: hsl(218deg 14% 39%); }
  .c82, .c82 a:link, .c82 a:visited { color: hsl(218deg 14% 37%); }
  .c88, .c88 a:link, .c88 a:visited { color: hsl(218deg 14% 37%); }
  .c9c, .c9c a:link, .c9c a:visited { color: hsl(218deg 14% 34%); }
  .cae, .cae a:link, .cae a:visited { color: hsl(218deg 14% 31%); }
  .cbe, .cbe a:link, .cbe a:visited { color: hsl(218deg 14% 28%); }
  .cce, .cce a:link, .cce a:visited { color: hsl(218deg 14% 26%); }
  .cdd, .cdd a:link, .cdd a:visited, .comment { color: hsl(218deg 14% 24%); }
}