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.flex-direction-nav a:before  { 
    content: " ";
    display: block;
    background: url('img/prev.png') no-repeat center center;
    background-size: contain;
    width: 40px;
    height: 40px;
}
.flex-direction-nav a.flex-next:before  { 
    content: " ";
    display: block;
    background: url('img/next.png') no-repeat center center;
    background-size: contain;
    width: 40px;
    height: 40px;
}