Social Theme - Where to add to the child theme css

I have figured out where to add some random code to show up in the main area of the BP Social theme, but this is by changing the CSS in the parent theme.

If I was to edit the child theme CSS, how would I edit it?

The section I changed is the base.css at line 6, and I have the following code:

background: url(/wp-content/themes/bp-social-child/_inc/images/bg/rotate.php) no-repeat;

This shows up a random image as the background, as you can see here.