You should always create a child theme before modifying your sites CSS.
Aim: To achieve the results you can see on this page, in this case, a stainless steel background image on the content area.
Set the id #main to your background image then set the various classes areas making up the Twenty Fourteen content area to transparent allowing the background image to show through on your WordPress website page.
#main { background-image:url('/img/yourimage.jpg');} #main, .site-content .entry-header, .site-content .entry-content, .site-content .entry-meta, .page-content { background-color:transparent; }