How to Hide Cell Borders for the Main Menu

Hello,
add in your style.css (child theme)

header nav .nav li a { 
border-right: none;
}

You will find that the Modern theme has already this as standard in the child CSS:

header nav .nav li a {
border-right: 1px solid rgba( 255, 255, 255, 0.15 );
padding: 10px;
}

header nav ul li:first-child a {
border-left: 1px solid rgba( 255, 255, 255, 0.15 );
padding-left: 1.5em;
}

Just adjust the current CSS instead of adding new code, is probably more advisable.

So to implement Simone’s advice, change the above to

header nav .nav li a {
border-right: none;
padding: 20px;
}

header nav ul li:first-child a {
border-left: none;
padding-left: 1.5em;
}