Over-riding base.css

Hi

I've created a child theme and enabled it.

I want to over-ride some settings in wp-content/themes/blogs-mu/_inc/css/base.css but it doesn't work when I add it to child-style.css (even when using !important)

I've also tried copying base.css to the identicle location in the child theme. That doesn't work either.

How do I accomplish this without relying on changes to the main theme?

Thanks so much!

Corey