Add .nav-secondary.sticky class and make changes to style.css

.admin-bar .nav-secondary.fixed {
    top: 32px;
}

.nav-primary.fixed {
    position: fixed;
    top: 100px;
}

/* Adjusting the top margin for Firefox */
@-moz-document url-prefix() {
.nav-primary.fixed {
    position: fixed;
    top: 70px;
    }
}

/* Adjusting the top margin for Microsoft IE10+ and Edge */
_:-ms-lang(x), .nav-primary.fixed {
 position: fixed;
 top: 70px;
}

.nav-secondary.fixed {
    position: fixed;
    margin: 0 auto;
    width: 100%;
    z-index: 999;
}