web/_includes/comments/utterances.html

<!-- https://utteranc.es/ -->
<script
  src="https://utteranc.es/client.js"
  repo="{{ site.comments.utterances.repo }}"
  issue-term="{{ site.comments.utterances.issue_term }}"
  crossorigin="anonymous"
  async
></script>

<script type="text/javascript">
  (function () {
    const origin = 'https://utteranc.es';
    const iframe = 'iframe.utterances-frame';
    const lightTheme = 'github-light';
    const darkTheme = 'github-dark';
    let initTheme = lightTheme;
    const html = document.documentElement;

    if (
      (html.hasAttribute('data-mode') && html.getAttribute('data-mode') === 'dark') ||
      (!html.hasAttribute('data-mode') && window.matchMedia('(prefers-color-scheme: dark)').matches)
    ) {
      initTheme = darkTheme;
    }

    addEventListener('message', (event) => {
      let theme;

      /* credit to <https://github.com/utterance/utterances/issues/170#issuecomment-594036347> */
      if (event.origin === origin) {
        /* page initial */
        theme = initTheme;
      } else if (event.source === window && event.data && event.data.direction === ModeToggle.ID) {
        /* global theme mode changed */
        const mode = event.data.message;
        theme = mode === ModeToggle.DARK_MODE ? darkTheme : lightTheme;
      } else {
        return;
      }

      const message = {
        type: 'set-theme',
        theme: theme
      };

      const utterances = document.querySelector(iframe).contentWindow;
      utterances.postMessage(message, origin);
    });
  })();
</script>