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 (“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


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?