css overriding not working

I copied and pasted the better_messages.css file to my theme folder. Let's say my theme is named abc, then the css file is placed under the sites/all/themes/abc. What I was tring to accomplish is to change the background color of h2.error, h2.status, and h2.warning but it didn't work unless I made the change directly to the css file in the module folder. I cleared cache but still, wonder how to get the css file under the theme folder work.

Thanks