johnyc on "[Theme: Twentytwelve] full width header"

I've setup child theme for TwentyTwelve & I want to set width of header & nav-bar section to be full.
Just like what you see the desktop version of this WP Support Forums thread. The wrapper width is 960px but the header area with #222 black color extends goes to full width of the computer screen.
If I define width 100% it still stays at 960px (or whatever the container width is defined).