diff --git a/progress_bar.module b/progress_bar.module
index 677a045..1a30398 100644
--- a/progress_bar.module
+++ b/progress_bar.module
@@ -30,3 +30,17 @@ function progress_bar_page_attachments(array &$attachments) {
   // Unconditionally attach an css to the page.
   $attachments['#attached']['library'][] = 'progress_bar/progress-bar';
 }
+
+/**
+ * Implements hook_theme().
+ */
+function progress_bar_theme() {
+  return [
+    'progress_bar_format' => [
+      'variables' => array(
+        'state' => NULL,
+      ),
+      'template' => 'progress-bar-format',
+    ],
+  ];
+}