fix: check if DISQUS is loaded before accessing it (#626)

closes https://github.com/CaiJimmy/hugo-theme-stack/issues/571

layouts/partials/comments/provider/disqus.html

@@ -13,7 +13,7 @@
 
 <script>
     window.addEventListener('onColorSchemeChange', (e) => {
-        if (DISQUS) {
+        if (typeof DISQUS == 'object') {
             DISQUS.reset({
                 reload: true
             });