How do I customize plugins CSS?

This is a general question about customizing a CSS of a plugin, but I'll use a specific example.
I'm using the NinjaForms plugin in http://wisdom-edge.com/members/tal_dn/ ("Contribute" tab). In it I'd like to change the element
ul.term-tabs li {
background: #ccc; }
which appears in some css file of the plugin
to
ul.term-tabs li {
background: transparent; }

I added this latter code to style.css of my active child theme and it didn't change anything.

What am I doing wrong?
Thanks.