<nav>
  <ul class="nav-menu">
    <li>item</li>
    <li>item</li>
    <li>item</li>
  </ul>
</nav>

nav {
  width: 1200px;
  height: 50px;
  background: #f00;
  position: relative;
}
nav:before {
  content: '';
  display: block;
  position: absolute;
  width: 100vw;
  height: 100%;
  background: rgba(0,0,0,.5);
  z-index: 1;
}
.nav-menu {
  color: #fff;
  list-style: none;
  display: flex;
  justify-content: space-around;
  padding: 15px;
  position: relative;
  z-index: 2;
}
.nav-menu li {
  transition: opacity .5s;
}
.nav-menu li:hover {
  opacity: .8;
}