refactor(ui): reduce the size of the 'close' and 'copy' buttons

- The close button in the TOC popup
- The copy button in the code block

_layouts/post.html

@@ -113,9 +113,7 @@ script_includes:
     <dialog id="toc-popup" class="p-0">
       <div class="header d-flex flex-row align-items-center justify-content-between">
         <div class="label text-truncate py-2 ms-4">{{- page.title -}}</div>
-        <button id="toc-popup-close" type="button" class="btn mx-1 my-1 opacity-75">
-          <i class="fas fa-close"></i>
-        </button>
+        <button id="toc-popup-close" type="button" class="btn-close btn-sm mx-3" aria-label="Close"></button>
       </div>
       <div id="toc-popup-content" class="px-4 py-3 pb-4"></div>
     </dialog>