Refactor CSS class names for consistency and clarity

- Update selectors from IDs to classes in header, search, and zmedia styles.
- Ensure consistent usage of class names across relevant files.

layouts/_default/search.html

@@ -20,10 +20,10 @@
     {{- end }}
 </header>
 
-<div id="searchbox">
+<div id="searchbox" class="searchbox">
     <input id="searchInput" autofocus placeholder="{{ .Params.placeholder | default (printf "%s ↵" .Title) }}"
         aria-label="search" type="search" autocomplete="off" maxlength="64">
-    <ul id="searchResults" aria-label="search results"></ul>
+    <ul id="searchResults" class="searchResults" aria-label="search results"></ul>
 </div>
 
 {{- end }}{{/* end main */}}