Override the thing that's extended

agaric/templates/field/field--paragraph--text--field-text--preview.html.twig

@@ -1 +1,19 @@
-{% extends "field--text.html.twig" %}
+{% extends "field.html.twig" %}
+{#
+/**
+ * @file
+ * Default theme implementation for a text field.
+ *
+ * A 'content' class is added to provide default styling of base elements such
+ * as paragraphs and lists that may not have classes assigned to them. This
+ * allows user entered content to have default styling without interfering with
+ * the styles of other UI components such as system generated lists or other
+ * dynamic content.
+ *
+ * @see https://www.drupal.org/node/2539860
+ * @see http://bulma.io/documentation/elements/content
+ *
+ * @ingroup themeable
+ */
+#}
+{% set attributes = attributes.addClass('content') %}