Hello,
Why sprite? Try with this in your custom css or child-theme style.css:
.sidebar h3 {
background: #373636;
background: -moz-linear-gradient(top, #373636 0%, #262525 100%);
background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,#373636), color-stop(100%,#262525));
background: -webkit-linear-gradient(top, #373636 0%,#262525 100%);
background: -o-linear-gradient(top, #373636 0%,#262525 100%);
background: -ms-linear-gradient(top, #373636 0%,#262525 100%);
background: linear-gradient(to bottom, #373636 0%,#262525 100%);
filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#373636', endColorstr='#262525',GradientType=0 );
border-left: 4px solid #C93603;
border-radius:none !important;
}