How to Override Module CSS in Custom Theme?

Arron's comment actually was correct. I just didn't know I needed to clear pub/static and var/view_preprocessed.

To override this file in your theme:

app/code/Ubertheme/UbContentSlider/view/frontend/web/css/owl-carousel1/owl.theme.css

You would place your custom owl.theme.css here:

app/design/frontend/[Namespace]/[theme]/Ubertheme_UbContentSlider/web/css/owl-carousel1/owl.theme.css

Don't forget to run "rm -rf pub/static/* var/view_preprocessed/*" !