extension.js

/**
 * Credits to 'deleerium' from HN.
 * @link https://news.ycombinator.com/item?id=23199649
 */
const CSS = `body { background-color: #1F2430; }
  #hnmain { background-color: #232834; }

  a:link.storylink,
  #hnmain > tbody > tr:first-child > td a,
  .commtext, .commtext a:link, td, b {
    color: #fafafa;
  }

  .comment > .commtext {
    color: #fafafa;
  }

  td > .title {
    color: #fafafa;
  }

  #hnmain > tbody > tr:first-child > td { background-color: #333a4a; }

  a:link,
  .sitebit, .subtext, .sitestr, .subtext a:link,
  .rank, a.morelink,
  .pagetop, .yclinks a, .yclinks > a:link,
  .comhead a:link, .comhead,
  .hnmore a:link, .reply a:link, {
    color: #8c96ac;
  }
  td .pagetop {
    color: #8c96ac;
  }
  td > a:link {
    color: #8c96ac;
  }
  .title > a:link {
    color: #8c96ac;
  }
  a > u {
    color: #8c96ac;
  }

  .comment .reply u > a {
    color: #8c96ac;
  }

  .title > a:visited {
    color: #979cf4;
  }

  .commtext > p > a:link {
    color: #fafafa;
  }
  .comment .commtext > a:link {
    color: #fafafa;
  }
  .comment .commtext > a:visited {
    color: #979cf4;
  }
  .commtext > p > a:visited {
    color: #979cf4;
  }

  .votearrow {
    overflow: visible;
    position: relative;
  }

  .yclinks > a, .yclinks > a:link {
    color: #8c96ac;
  }
  .yclinks > a:visited {
    color: #979cf4;
  }

  .votearrow::before {
    content: "";
    width: 0;
    height: 0;
    left: 1px;
    top: 3px;
    display: block;
    position: absolute;
    border-left: 4px solid transparent;
    border-right: 4px solid transparent;
    border-bottom: 7px solid #bebfd1;
  }

  input[type=text],
  textarea {
    background-color: #333a4a;
    color: #fafafa;
    border: 1px solid #1F2430;
  }

  input[type=submit] {
    background-color: #333a4a;
    color: #fafafa;
    border: 1px solid #1F2430;
  }

  b {
    color: #fafafa;
  }
}
`;

chrome.tabs.onUpdated.addListener(function ( tabId, info ) {
  if ( info.status === "complete" ) {
    chrome.tabs.get( tabId, function ( tab ) {
      if (isHackerNews( tab.url ) ) {
        chrome.tabs.insertCSS( tab.id, { code: CSS } );
      }
    });
  }
});

function isHackerNews( url ) {
  return new URL( url ).host === "news.ycombinator.com";
}