Improve dark theme button's accessibility

Signed-off-by: Harsh Shandilya <me@msfjarvis.dev>

layouts/partials/sidebar.html

@@ -16,7 +16,7 @@
       </ul>
     </nav>
     <div class="user-toggle">
-    <button class="toggle-button js-mode-toggle">
+    <button class="toggle-button js-mode-toggle" aria-label="Dark theme switch">
       <span class="toggle-button__icon" aria-hidden="true"></span>
     </button>
   </div>