fix: utterances script to obey the follow_site_theme setting (#91)

* Fix js for correct theme load

* Use dataTheme directly instead ternary operator

_includes/extensions/comments/utterances.html

@@ -1,46 +1,51 @@
-<script async
-        crossorigin="anonymous"
-        issue-term="{{ site.utterances.issue_term }}"
-        label="{{ site.utterances.label }}"
-        {%- if site.utterances.follow_site_theme -%}
-        onload="initUtterancesTheme()"
-        {%- endif -%}
-        repo="{{ site.utterances.repo }}"
-        src="https://utteranc.es/client.js"
-        theme="{{ site.utterances.theme }}">
-</script>
-
 {%- if site.utterances.follow_site_theme -%}
+<div id="utterances-placeholder"></div>
 <script>
-  const setUtterancesTheme = (dataTheme) => {
+  const utterancesThemeFromDataTheme = () => {
+    const dataTheme = document.documentElement.getAttribute('data-theme');
+    return `github-${dataTheme}`;
+  };
+
+  const setUtterancesTheme = () => {
     const iframe = document.querySelector('.utterances-frame');
     if (iframe) {
-      const utterancesTheme = dataTheme === 'dark' ? 'github-dark' : 'github-light'
       const message = {
         type: 'set-theme',
-        theme: utterancesTheme
+        theme: utterancesThemeFromDataTheme()
       };
       iframe.contentWindow.postMessage(message, 'https://utteranc.es');
     }
   }
 
-  // init theme
-  const initUtterancesTheme = () => {
-    const iframe = document.querySelector('.utterances-frame');
-    if (!iframe) return
-    iframe.onload = () => {
-      setUtterancesTheme(document.documentElement.getAttribute('data-theme'))
-    }
-  }
-
   // dynamic change
   const observer = new MutationObserver((mutationsList, observer) => {
     for (let mutation of mutationsList) {
       if (mutation.type === 'attributes' && mutation.attributeName === 'data-theme') {
-        setUtterancesTheme(document.documentElement.getAttribute('data-theme'))
+        setUtterancesTheme();
       }
     }
   });
   observer.observe(document.documentElement, { attributes: true, childList: false, subtree: false });
+
+  let utterancesScript = document.createElement('script');
+  utterancesScript.async = true;
+  utterancesScript.src = 'https://utteranc.es/client.js';
+  utterancesScript.crossOrigin = 'anonymous';
+  utterancesScript.setAttribute('issue-term', '{{ site.utterances.issue_term }}');
+  utterancesScript.setAttribute('label', '{{ site.utterances.label }}');
+  utterancesScript.setAttribute('repo', '{{ site.utterances.repo }}');
+  utterancesScript.setAttribute('theme', utterancesThemeFromDataTheme());
+
+  const placeholder = document.getElementById('utterances-placeholder');
+  placeholder.parentNode.replaceChild(utterancesScript, placeholder);
+</script>
+{%- else -%}
+<script async
+        crossorigin="anonymous"
+        issue-term="{{ site.utterances.issue_term }}"
+        label="{{ site.utterances.label }}"
+        repo="{{ site.utterances.repo }}"
+        src="https://utteranc.es/client.js"
+        theme="{{ site.utterances.theme }}">
  </script>
 {%- endif -%}