Custom header design

Swap the CSS for this:

/* Mobile border */
.main-navigation .inside-navigation {
    border-bottom: 1px solid #ccc;;
}
/* Desktop Border */
@media(min-width: 769px) {
    .main-navigation .inside-navigation {
        border-top: 1px solid #ccc;
        border-bottom: none;
    }
}

Hello,

Can u please confirm?

I've to remove the old below one CSS and then I add the new one. Right?

.main-navigation .inside-navigation {
border-top: 1px solid #ccc;
}

Thats correct.