diff --git a/core/modules/system/templates/system-plugin-ui-form.html.twig b/core/modules/system/templates/system-plugin-ui-form.html.twig
new file mode 100644
index 0000000..5019164
--- /dev/null
+++ b/core/modules/system/templates/system-plugin-ui-form.html.twig
@@ -0,0 +1,31 @@
+{#
+/**
+ * @file
+ * Default theme implementation to configure blocks.
+ *
+ * Available variables:
+ * - $left: Any form array elements that should appear in the left hand column.
+ * - $right: Any form array elements that should appear in the right hand column.
+ * - $form_submit: Form submit button.
+ *
+ * @see template_preprocess_block_library_form()
+ * @see theme_block_library_form()
+ *
+ * @ingroup themeable
+ */
+#}
+<div id="block-library" class="container">
+  <div class="left-col">
+    <div class="inside">
+      {{ left }}
+    </div>
+  </div>
+  <div class="right-col">
+    <div class="inside">
+      {{ right }}
+    </div>
+  </div>
+  {% if form_submit %}
+  <div class="bottom-bar">{{ form_submit }}</div>
+  {% endif %}
+</div>
diff --git a/core/modules/system/templates/system-plugin-ui-form.tpl.php b/core/modules/system/templates/system-plugin-ui-form.tpl.php
deleted file mode 100644
index 8179076..0000000
--- a/core/modules/system/templates/system-plugin-ui-form.tpl.php
+++ /dev/null
@@ -1,32 +0,0 @@
-<?php
-
-/**
- * @file
- * Default theme implementation to configure blocks.
- *
- * Available variables:
- * - $left: Any form array elements that should appear in the left hand column.
- * - $right: Any form array elements that should appear in the right hand column.
- * - $form_submit: Form submit button.
- *
- * @see template_preprocess_block_library_form()
- * @see theme_block_library_form()
- *
- * @ingroup themeable
- */
-?>
-<div id="block-library" class="container">
-  <div class="left-col">
-    <div class="inside">
-      <?php print $left; ?>
-    </div>
-  </div>
-  <div class="right-col">
-    <div class="inside">
-      <?php print $right; ?>
-    </div>
-  </div>
-  <?php if ($form_submit) { ?>
-  <div class="bottom-bar"><?php print $form_submit; ?></div>
-  <?php } ?>
-</div>