[BS5] add theme selector

data/web/templates/admin/tab-config-customize.twig

@@ -67,6 +67,14 @@
       <legend data-bs-target="#ui_texts" style="padding-top:20px" unselectable="on">{{ lang.admin.ui_texts }}</legend><hr />
       <div id="ui_texts">
         <form class="form" data-id="uitexts" role="form" method="post">
+          <div class="mb-4 d-flex flex-column">
+            <label for="uitests_theme">{{ lang.admin.ui_theme }}:</label>
+            <select class="full-width-select" data-live-search="true" id="uitests_theme" name="ui_theme" required>
+              {% for theme in themes %}
+              <option>{{ theme }}</option>
+              {% endfor %}
+            </select>
+          </div>
           <div class="mb-2">
             <label for="uitests_title_name">{{ lang.admin.title_name }}:</label>
             <input type="text" class="form-control" id="uitests_title_name" name="title_name" placeholder="mailcow UI" value="{{ ui_texts.title_name|raw }}">