Change top bar (navigation) bar width

Hi There,

I would like to make my site 80% width of the page. I have successfully make the content, footer, and header to that width.
However, I could not figure out the proper way to make the .head ‘s width fit like the content and footer.
What I mean is, I would like to make the .head (the navigation bar contain logo and menu) 80% width and the rest 20% as the same color as the page background color.
Below is a screenshoot uploaded to google drive explain my question:
https://drive.google.com/open?id=1e271e96MUgPpeyH71yO_RJPA1B–Ti5a
Wish to get any help and every help is appreciated.

Thanks & Regards,

The page I need help with: [log in to see the link]