Hi Beatriz,
With regards to the first issue, here is a CSS you may try:
@media (max-width: 768px) {
.main-navigation .main-nav ul li a {
background-color:rgb(255,255,255);
}
.main-navigation .main-nav > ul > li > a {
border-bottom: solid 1px black;
}
}
With regards to the second issue:
@media (min-width: 769px){
.search nav#site-navigation:after {
content: "";
display: block;
margin-top:5px;
border-bottom: 1px solid rgb(0,0,0);
}
}
Adding these through Appearance > Customize > Additional CSS should work.
If anything doesn’t adhere to your preference, may you kindly send a screenshot of a layout of what it should look like?
Kindly let us know how it goes. 🙂