Where Is This Css Selector???

So I am having some issues with the mesocolum theme, basically, I want to add an opacity to the main content wrapper.

.container-wrap I can find it in Firebug, I can even find it in the css file, but when I apply my style it will not work.

Thing is that when I find the .container-wrap in the styles.css file, it does not declare the background-color???

Could someone help me with this please?

Thanks,

Stephen.