Append this to news.css;

  @media (prefers-color-scheme: dark) {
      
      body { background-color:#000; }
      #hnmain { background-color:#000; }
      
      a:link { color:#eee; }
      a:visited { color:#b2b2b2; }
      
      .default, .title, .subtext, .yclinks, .comhead { color:#a2a2a2; }
      .admin { color:#eee; }
      .pagetop { color:#222; } /* not inverted */
      
      .c00, .c00 a:link { color:#eee; }
      .c5a, .c5a a:link, .c5a a:visited { color:#a5a5a5; }
      .c73, .c73 a:link, .c73 a:visited { color:#8c8c8c; }
      .c82, .c82 a:link, .c82 a:visited { color:#7d7d7d; }
      .c88, .c88 a:link, .c88 a:visited { color:#777777; }
      .c9c, .c9c a:link, .c9c a:visited { color:#636363; }
      .cae, .cae a:link, .cae a:visited { color:#515151; }
      .cbe, .cbe a:link, .cbe a:visited { color:#414141; }
      .cce, .cce a:link, .cce a:visited { color:#313131; }
      .cdd, .cdd a:link, .cdd a:visited { color:#222222; }
      
      .pagetop a:link, .pagetop a:visited { color: #000; } /* not inverted */
      .topsel a:link, .topsel a:visited { color:#000; }
      .subtext a:link, .subtext a:visited { color:#7d7d7d; }
      .comhead a:link, .subtext a:visited { color:#7d7d7d; }
      .hnmore a:link, a:visited { color:#7d7d7d; }
      
      textarea { background-color: #000; color: #eee; }
      
  }

Inverted most colours (ff - 5a = a5).

#eee for text colour.

#000 for bg (body, #hnmain)

#hnmain's background is set with an HTML attribute, !important might be necessary for some browsers? Not sure, but works on my machine(tm).

Edit: s/html/body/, wasn't necessary to select on <html>. Split body and #hnmain selectors.