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

assets/css/jekyll-theme-chirpy.scss

@@ -1,6 +1,10 @@
 ---
 ---
 
+@use 'abstracts/variables' with (
+  $theme: '{{ site.theme_mode }}'
+);
+
 /* prettier-ignore */
 @use 'main
 {%- if jekyll.environment == 'production' -%}