Remove card styling from ngx-bootstrap accordion

I am using ngx-bootstrap accordion with angular 9. The header always contains the 'card' and 'card-header' classes and I don't want them .. how can I remove these classes when the html is constructed

this is my code:

  <accordion>
  <accordion-group>
  <div accordion-heading>
          'HEADER TEXT'
      </div>
      <div>
          'BODY TEXT'
      </div>
  </accordion-group>
</accordion>

And what I get is

<accordion-group _ngcontent-yeu-c53="" class="panel panel-open" style="display: block;" _nghost-yeu-c13="" ng-reflect-is-open="true"><div _ngcontent-yeu-c13="" class="panel card panel-default" ng-reflect-ng-class="panel-default"><div _ngcontent-yeu-c13="" role="tab" class="panel-heading card-header panel-enabled" ng-reflect-ng-class="panel-enabled">