diff --git a/core/modules/block/src/BlockForm.php b/core/modules/block/src/BlockForm.php
index 248edf7..4eff702 100644
--- a/core/modules/block/src/BlockForm.php
+++ b/core/modules/block/src/BlockForm.php
@@ -140,6 +140,7 @@ public function form(array $form, FormStateInterface $form_state) {
       );
     }
     else {
+      $theme = $this->config('system.theme')->get('default');
       $theme_options = array();
       foreach ($this->themeHandler->listInfo() as $theme_name => $theme_info) {
         if (!empty($theme_info->status)) {