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/sidebar.html

@@ -40,11 +40,53 @@
     </ul>
   </nav>
 
-  <div class="sidebar-bottom d-flex flex-wrap  align-items-center w-100">
-    {% unless site.theme_mode %}
-      <button type="button" class="btn btn-link nav-link" aria-label="Switch Mode" id="mode-toggle">
-        <i class="fas fa-adjust"></i>
-      </button>
+  <div class="sidebar-bottom d-flex flex-wrap align-items-center w-100">
+    {% unless site.theme_mode == 'light' or site.theme_mode == 'dark' %}
+      {%- capture icon_system -%}
+        <i class="fa-solid fa-display" data-theme-mode="system"></i>
+      {%- endcapture -%}
+
+      {%- capture icon_light -%}
+        <i class="fa-regular fa-sun" data-theme-mode="light"></i>
+      {%- endcapture -%}
+
+      {%- capture icon_dark -%}
+        <i class="fa-regular fa-moon" data-theme-mode="dark"></i>
+      {%- endcapture -%}
+
+      <div class="btn-group dropup">
+        <button
+          type="button"
+          class="btn btn-link nav-link"
+          aria-label="Switch Mode"
+          id="mode-toggle"
+          data-bs-toggle="dropdown"
+        >
+          {{- icon_light -}}
+          {{- icon_dark -}}
+          {{- icon_system -}}
+        </button>
+        <ul class="dropdown-menu rounded-3 mb-1 p-1">
+          <li>
+            <button class="dropdown-item d-flex align-items-center" type="button" data-theme-mode="light">
+              {{- icon_light -}}
+              {{- site.data.locales[lang].theme.light -}}
+            </button>
+          </li>
+          <li>
+            <button class="dropdown-item d-flex align-items-center" type="button" data-theme-mode="dark">
+              {{- icon_dark -}}
+              {{- site.data.locales[lang].theme.dark -}}
+            </button>
+          </li>
+          <li>
+            <button class="dropdown-item d-flex align-items-center" type="button" data-theme-mode="system">
+              {{- icon_system -}}
+              {{- site.data.locales[lang].theme.system -}}
+            </button>
+          </li>
+        </ul>
+      </div>
 
       {% if site.data.contact.size > 0 %}
         <span class="icon-border"></span>