Twenty Sixteen adjust white space left and right of body

 /* Adjust width, add colour to body custom background and make responsive. Up to 14% keeps 1200 pixel width of site */

@media screen and (min-width: 56.875em) {
body.custom-background {
    margin-left: 12% !important;
    margin-right: 12% !important;
    background-color: #e5edcd !important;
    }
}

/* Add radius to background for rounded corners */

@media screen and (min-width: 56.875em) {
.site {
    border-radius: 30px;
    }
}