Add a drop shadow to a top-fixed nav bar using Bootstrap

I'm attempting to add a drop shadow to my nav bar I have through Bootstrap, but I can't seem to figure it out. Maybe somebody can point me in the correct direction. All the other questions I've seen that have been asked about a similar topic have incredibly lengthy responses of messy code, but there has to be a simple way to do it, isn't that the whole point of Bootstrap after all.

Here's my HTML relating to my nav:

<div class="navbar navbar-inverse navbar-fixed-top" id="custom-nav">
  <div class="navbar-inner">
    <div class="container">
      <a class="btn btn-navbar" data-toggle="collapse" data-target=".nav-collapse">
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
      </a>
      <a class="brand" href="index.html">eService</a>
      <div class="nav-collapse">
        <ul class="nav pull-right">
          <li><a href="index.html">Home</a></li>
          <li><a href="#about">About</a></li>
          <li><a href="contact.html">Contact</a></li>
          <li class="dropdown">
            <a href="#" class="dropdown-toggle" data-toggle="dropdown">
                  Services
                  <b class="caret"></b>
            </a>
            <ul class="dropdown-menu">
              <li><a href="#">Stage Design & Build</a></li>
              <li><a href="#">Event Planning & Management</a></li>
              <li><a href="#">Video Production</a></li>
              <li><a href="#">Audio Production</a></li>
              <li><a href="#">Lighting Production</a></li>
            </ul>
          </li><!-- dropdown -->
          <li clas