How to add or remove borders in navigation menu?

With CSS 3, you can use the :last-child or :last-of-type selectors:

.is-frontend #content-holder .semplice-navbar nav ul li:last-child {
border-right: 1px solid #000000;
}

However, you could flip this around to make it more backwards compatible using :first-child (which is available in CSS2):

.is-frontend #content-holder .semplice-navbar nav ul li {
border-left: 1px solid #000000;
}

.is-frontend #content-holder .semplice-navbar nav ul li:first-child {
border-right: none;
}