@media only screen and (max-width: 990px) {
.navbar-nav {
    text-align:center;
}
.navbar-nav>li {
    display: inline-block;
    width: auto;
}
.navbar-nav>li a {
    display: inline-block;
    width: auto;
}

@media only screen and (max-width: 770px) {
.navbar-nav>li {
    display: block;
    width: 100%;
}
.navbar-nav>li a {
    display: block;
    width: 100%;
}

}