diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 0d472e5..ea819f3 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -2690,9 +2690,6 @@ function system_preprocess_block(&$variables) {
  */
 function template_preprocess_system_plugin_ui_form(&$variables) {
   drupal_add_css(drupal_get_path('module', 'system') . '/system.plugin.ui.css');
-  $variables['left'] = drupal_render($variables['form']['left']);
-  $variables['right'] = drupal_render($variables['form']['right']);
-  $variables['form_submit'] = drupal_render_children($variables['form']);
 }
 
 /**
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..f5b473c
--- /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:
+ * - form: The form.
+ * - form.left: Form elements that appear in the left column.
+ * - form.right: Form elements that appear in the right column.
+ *
+ * @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">
+      {{ form.left }}
+    </div>
+  </div>
+  <div class="right-col">
+    <div class="inside">
+      {{ form.right }}
+    </div>
+  </div>
+  {% if form_submit -%}
+    <div class="bottom-bar">{{ form }}</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>