.bg-dark{
    text-align: right;
}
.bg-dark ul li{
    margin: 0 20px;
}
.bg-dark ul li::after{
    content: '';
    height: 3px;
    width: 0;
    background: #009688;
    left: 0;
    bottom: -10px;
    transition: 0.5s;
}
.bg-dark ul li:hover::after{
    width:100%;
}