[Theme: Twentytwelve] full width header

Hello, I've setup a child theme with 'twentytwelve' as a parent theme. I would want my header & nav-bar to utilise full width stretching to end as per browser-window/screen width. It should follow fluid width when I put 100% as width parameter. But it restricts to 960px (or whatever set) the width of wrapper/ container for twentytwelve.

What is work-around for this? TwentyTwelve theme comes bundled with WP installation so finding it's CSS file would not be a problem. (still here's the source http://phpxref.ftwr.co.uk/wordpress/nav.html?wp-content/themes/twentytwelve/style.css.source.html )
TwentyTwelve is mobile friendly responsive theme & I dont want to ruin that.
Attached is the screenshot to demonstrate the idea..
As seen in screenshot attached below the header & nav-bar areas are using full width of the browser whereas container/body has fixed width.
... something similar I want to implement on my child theme.
Thank you.