Hi there,
You can add this following css on child theme style.css or additional css section of customizer[if you are using WordPress version 4.7]
#content-wrapper {
background-color: red !important;
}
Note: Now background-color value is red please change the background-color value as per your requirement.YOu can see lost of color value here: http://www.w3schools.com/cssref/css_colors.asp
Thanks
