Make layout more compact

src/Plugin/Block/ArchiveTreeBlock.php

@@ -89,7 +89,7 @@ class ArchiveTreeBlock extends BlockBase {
     }
 
     return [
-      '#markup' => $output,
+      '#markup' => '<div class="archive-tree-block">' . $output . '</div>',
       '#allowed_tags' => ['details', 'summary', 'a', 'div'],
       '#attached' => [
         'library' => [