Fix Add button style in GroupsEditor

Author: Giganzo

editor/groups_editor.cpp

@@ -840,7 +840,7 @@ GroupsEditor::GroupsEditor() {
 	add_child(hbc);
 
 	add = memnew(Button);
-	add->set_flat(true);
+	add->set_theme_type_variation("FlatMenuButton");
 	add->set_tooltip_text(TTR("Add a new group."));
 	add->connect(SceneStringName(pressed), callable_mp(this, &GroupsEditor::_show_add_group_dialog));
 	hbc->add_child(add);