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/head.html

@@ -81,7 +81,7 @@
 
   <!-- Bootstrap -->
   {% unless jekyll.environment == 'production' %}
-    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@5/dist/css/bootstrap.min.css">
+    <link rel="stylesheet" href="{{ site.data.origin.cors.bootstrap.css }}">
   {% endunless %}
 
   <!-- Theme style -->