<nav class="navbar navbar-expand-lg navbar-dark bg-dark">
  <div class="container-fluid"><a class="navbar-brand" href="#">Navbar</a><button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarDarkExample" aria-controls="navbarDarkExample" aria-expanded="false" aria-label="Toggle navigation"><span class="navbar-toggler-icon"></span></button>
    <div class="collapse navbar-collapse" id="navbarDarkExample">
      <ul class="navbar-nav mr-auto mb-2 mb-lg-0">
        <li class="nav-item"><a class="nav-link active" aria-current="page" href="#">Home</a></li>
        <li class="nav-item"><a class="nav-link" href="#">Link</a></li>
        <li class="nav-item dropdown"><a class="nav-link dropdown-toggle" href="#" id="navbarDarkExampleDropdown" role="button" data-toggle="dropdown" aria-expanded="false">Dropdown</a>
          <div class="dropdown-menu py-0" aria-labelledby="navbarDarkExampleDropdown">
            <div class="bg-white py-2"><a class="dropdown-item" href="#">Action</a><a class="dropdown-item" href="#">Another action</a>
              <hr class="dropdown-divider" /><a class="dropdown-item" href="#">Something else here</a>
            </div>
          </div>
        </li>
        <li class="nav-item"><a class="nav-link disabled" href="#" tabindex="-1" aria-disabled="true">Disabled</a></li>
      </ul>
      <form class="d-flex"><input class="form-control mr-2" type="search" placeholder="Search" aria-label="Search" /><button class="btn btn-outline-light" type="submit">Search</button></form>
    </div>
  </div>
</nav>

<nav class="navbar navbar-expand-lg navbar-dark bg-primary">
  <div class="container-fluid"><a class="navbar-brand" href="#">Navbar</a><button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarPrimaryExample" aria-controls="navbarPrimaryExample" aria-expanded="false" aria-label="Toggle navigation"><span class="navbar-toggler-icon"></span></button>
    <div class="collapse navbar-collapse" id="navbarPrimaryExample">
      <ul class="navbar-nav mr-auto mb-2 mb-lg-0">
        <li class="nav-item"><a class="nav-link active" aria-current="page" href="#">Home</a></li>
        <li class="nav-item"><a class="nav-link" href="#">Link</a></li>
        <li class="nav-item dropdown"><a class="nav-link dropdown-toggle" href="#" id="navbarPrimaryExampleDropdown" role="button" data-toggle="dropdown" aria-expanded="false">Dropdown</a>
          <div class="dropdown-menu py-0" aria-labelledby="navbarPrimaryExampleDropdown">
            <div class="bg-white py-2"><a class="dropdown-item" href="#">Action</a><a class="dropdown-item" href="#">Another action</a>
              <hr class="dropdown-divider" /><a class="dropdown-item" href="#">Something else here</a>
            </div>
          </div>
        </li>
        <li class="nav-item"><a class="nav-link disabled" href="#" tabindex="-1" aria-disabled="true">Disabled</a></li>
      </ul>
      <form class="d-flex"><input class="form-control mr-2" type="search" placeholder="Search" aria-label="Search" /><button class="btn btn-outline-light" type="submit">Search</button></form>
    </div>
  </div>
</nav>