Child CSS In Studio Theme

Hi guys,

Just working out how child CSS works in Studio.

Say I want to change the black background on the header of Studio.

The default CSS is
#header-wrapper {
background: none repeat scroll 0 0 #272727;
}

Now I want to change it to white.
So if I add

#header-wrapper {
background: none repeat scroll 0 0 #FFFFFF;
}

In the child CSS shouldn't it change it for me?

I have done but to no avail. I have noticed that you have to comment out a line in style.css in order to use child.css and I have done that also.
Where am I going wrong.

The site is http://www.mytestspace.me/test