The CSS code you provided was NOT in the Custom CSS as you mentioned. Regardless, I applied the code myself and it's still not working.
The CSS is being stripped in tablet and mobile view, but works fine in Desktop view.
When I inspect the element it's now showing this
@media (min-width: 1400px)
@media (min-width: 320px)
.navborderbottom {
border-bottom: 1px solid #D2D2D2;
width: 85%;
}
As you can see it's still forcing the min-width:1400px media query even if I try to add 320px. Again I ask you...why is it automatically applying the media query to my custom CSS?