gistfile1.css

$theme.slider .sc-handle { /* the knob */
    background-image: sc_static('images/knob.png') !important;

    @media (-webkit-min-device-pixel-ratio:2) {
        background-image: sc_static('images/knob_retna.png') !important;
        background-size: 16px 18px;
    }

    &.active {
        background-image: sc_static('images/knob_active.png') !important;

        @media (-webkit-min-device-pixel-ratio:2) {
            background-image: sc_static('images/knob_active_retna.png') !important;
        }
    }    
}