Overlay onclick

basic.html

<nav class="navbar navbar-expand-lg navbar-dark bg-primary">
  <a class="navbar-brand" href="#">Brand</a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#main_nav">
    <span class="navbar-toggler-icon"></span>
  </button>
<div class="collapse navbar-collapse" id="main_nav">
    <ul class="navbar-nav">
      <li class="nav-item active"> <a class="nav-link" href="#">Home </a> </li>
      <li class="nav-item"><a class="nav-link" href="#"> About </a></li>
      <li class="nav-item"><a class="nav-link" href="#"> Services </a></li>
      <li class="nav-item dropdown">
        <a class="nav-link  dropdown-toggle" href="#" data-toggle="dropdown">  More items  </a>
          <ul class="dropdown-menu">
          <li><a class="dropdown-item" href="#"> Submenu item 1</a></li>
          <li><a class="dropdown-item" href="#"> Submenu item 2 </a></li>
          <li><a class="dropdown-item" href="#"> Submenu item 3</a></li>
          <li><a class="dropdown-item" href="#"> Submenu item 4 </a></li>
          </ul>
      </li>
    </ul>
</div> <!-- navbar-collapse.// -->
</nav>

style.css

@media all and (min-width:991px) {  
  .dropdown:after{
      content: ''; 
      top: 0;
      right: 0;
      bottom: 0;
      left: 0;
      transition: opacity .15s ease-in-out;
      opacity: 0;
      visibility:hidden;
      background-color: rgba(0,0,0,.7);
      pointer-events: none;
  }
  .dropdown.show .nav-link{
	position:relative;
	z-index:12;
  }
  .dropdown.show:after{
	z-index:10; opacity:1; 
	visibility:visible;
	position: fixed;
  }
}