Hi Alain. This css should get you pretty close. Other than that you’d need some custom javascript which, as Tom indicated, is beyond the scope of the forum:
.main-navigation {
background: #fff;
}
.main-navigation .main-nav ul li a {
color: #000;
}
.main-navigation .main-nav ul li a:hover,
.main-navigation .main-nav ul li.current-menu-item a,
.main-navigation .main-nav ul li.current-menu-item a:hover {
background: #fff;
color: #009ae1;
border-top: 3px solid #009ae1;
margin-top: -3px;
}
Adding CSS: https://generatepress.com/knowledgebase/adding-css/