Fix #476 broken darktheme embeds

One tiny step toward #419 as well ;p

templates/partials/dark_theme.html

@@ -1,7 +1,7 @@
 {% load static %}
 <script>
 if({{ request.user.dark_theme|lower|default:'false' }} || window.matchMedia('(prefers-color-scheme: dark)').matches) {
-    $('<link>').prependTo('head').attr({type : 'text/css', rel : 'stylesheet'}).attr('href', '{% static "css/dark_screen.css" %}');
+    document.querySelector('head').innerHTML += '<link rel="stylesheet" href="{% static "css/dark_screen.css" %}" type="text/css"/>';
     document.body.setAttribute('data-theme', 'dark');
 }
 </script>