Responsive background image in Row

Hello, I’ve seen a previous thread concerning this topic but the solution suggested is not working for me. I want to make the image in a row responsive. Row layout : full width stretched.
I try to change the CSS of : #panel-17-2-0-0> .panel-widget-style from background-size: cover to 100%. That could fixe the problem but the CSS that I publish in child theme does not overwrite the CSS.
Is there a way for me to do that?
Thanks a lot for your help.