simplify theme preloader

edit.html

@@ -72,7 +72,6 @@
     <script src="edit/codemirror-default.js"></script>
 
     <link rel="stylesheet" href="/edit/codemirror-default.css">
-    <link id="cm-theme" rel="stylesheet">
 
     <template data-id="appliesTo">
       <li>

edit/edit.js

@@ -92,14 +92,12 @@ function preinit() {
   });
 
   // preload the theme so that CodeMirror can calculate its metrics in DOMContentLoaded->setupLivePrefs()
-  new MutationObserver((mutations, observer) => {
-    const themeElement = $('#cm-theme');
-    if (themeElement) {
-      themeElement.href = prefs.get('editor.theme') === 'default' ? ''
-        : 'vendor/codemirror/theme/' + prefs.get('editor.theme') + '.css';
-      observer.disconnect();
-    }
-  }).observe(document, {subtree: true, childList: true});
+  document.head.appendChild(
+    $create('link#cm-theme', {
+      rel: 'stylesheet',
+      href: prefs.get('editor.theme') === 'default' ? '' :
+        'vendor/codemirror/theme/' + prefs.get('editor.theme') + '.css'
+    }));
 
   if (chrome.windows) {
     queryTabs({currentWindow: true}).then(tabs => {