feat(theme): persist user theme preferences (#2756)

- Migrate theme persistence from `sessionStorage` to `localStorage`
- Rename theme HTML attribute to `data-bs-theme` for Bootstrap compatibility
- Trim and compile CSS according to the chosen theme mode

_includes/comments/disqus.html

@@ -19,7 +19,7 @@
 
   {%- comment -%} Auto switch theme {%- endcomment -%}
   function reloadDisqus(event) {
-    if (event.source === window && event.data && event.data.id === Theme.ID) {
+    if (event.source === window && event.data && event.data.id === Theme.eventId) {
       {%- comment -%} Disqus hasn't been loaded {%- endcomment -%}
       if (typeof DISQUS === 'undefined') {
         return;
@@ -33,7 +33,7 @@
 
   addDisqus();
 
-  if (Theme.switchable) {
+  if (Theme.isToggleable) {
     addEventListener('message', reloadDisqus);
   }