Actualización

app/config/mopa/mopa_bootstrap.yml

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+# see https://github.com/phiamo/MopaBootstrapBundle/blob/master/Resources/doc/misc/configuration-reference.md
+mopa_bootstrap:
+    # Adds twig form theme  support
+#        horizontal_label_class: 'aa'
+#        horizontal_input_wrapper_class: 'dd'
+    menu: ~  # enables twig helpers for menu
+    form:
+          show_legend: true # default is true
+          show_child_legend: false # default is true
+          error_type: block # or inline which is default
+          render_optional_text: true
+
+          collection:
+              widget_remove_btn:
+                  icon: trash
+                  attr:
+                      class: btn btn-danger
+              widget_add_btn:
+                  icon: plus-sign
+                  attr:
+                      class: btn btn-primary