Extend header and footer to include background

Is there any way to extend both the ‘header’ and ‘footer’ so that they extend across the entire page, whilst keeping my main page width as it is, without using image files?

http://www.nhspropertyservices.co.uk

I would like to be able to extend the green menu bar and the dark grey footer bar, so that they extend across the full width of the screen, whilst keeping the actual main page width the same as it is currently, so in effect the green and grey bars will extend both left and right, whilst the navigation menu will remain exactly as it appears now.

I am new to both WP and CSS and so any help will be gratefully appreciated in terms of which parts of with the header and footer.php and style.css files need to be adjusted, assuming this is possible.

Many thanks