diff --git a/css/archive_tree.css b/css/archive_tree.css
index 8ba2413..70ae630 100644
--- a/css/archive_tree.css
+++ b/css/archive_tree.css
@@ -1,3 +1,11 @@
+.archive-tree-block details,
+.archive-tree-block .archive-tree-year,
+.archive-tree-block details > summary.archive-tree-year {
+  margin-top: 0;
+  margin-bottom: 0;
+  padding-top: 0;
+  padding-bottom: 0;
+}
 .archive-tree-year {
   font-weight: bold;
 }
diff --git a/src/Plugin/Block/ArchiveTreeBlock.php b/src/Plugin/Block/ArchiveTreeBlock.php
index 5241a78..9af9e79 100644
--- a/src/Plugin/Block/ArchiveTreeBlock.php
+++ b/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' => [