.topnav li a {
    text-decoration: none;
    position: relative;
    padding: 6px 0;
    -webkit-transition: all 0.8s;
    transition: all 0.8s;
    color: lightblue;
}

.topnav li a:after {
    content:'';
    width:0%;
    margin-left:50%;
    height:2px;
    bottom:0;
    left:0;
    border-radius:40%;
    position:absolute;
    transition:all 0.5s;
}

.topnav li a:hover:after {
    background-color:lightblue;
    width:60%;
    margin-left:20%;
    box-shadow:0px 0px 10px lightblue;
}

.topnav li a.active:after {
    background-color:lightblue;
    width:60%;
    margin-left:20%;
    box-shadow:0px 0px 10px lightblue;
}