Thank you Aditya, that was the piece I was missing. Just for other’s sake, here is the complete code to add to your child theme’s style.css for a gradient background in the Twenty Eleven Theme (change the color black #000000 to whatever color you want to be at the top and change the color white #ffffff to whatever color you want the be at the bottom).
/* =Gradient Background
———————————————————*/
html {
height: 100%;
}
body {
height: 100%;
margin: 0;
background-repeat: no-repeat;
background-attachment: fixed;
background: #000000;
background: -moz-linear-gradient(top, #000000 0%, #ffffff 100%);
background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,#000000), color-stop(100%,#ffffff));
background: -webkit-linear-gradient(top, #000000 0%,#ffffff 100%);
background: -o-linear-gradient(top, #000000 0%,#ffffff 100%);
background: -ms-linear-gradient(top, #000000 0%,#ffffff 100%);
background: linear-gradient(to bottom, #000000 0%,#ffffff 100%);
filter: progid:DXImageTransform.Microsoft.gradient( startColorstr=’#000000′, endColorstr=’#ffffff’,GradientType=0 );
}