Edit in JSFiddle

<!-- Navigation -->
<nav class="navbar navbar-expand-lg navbar-dark bg-dark static-top">
  <div class="container">
    <a class="navbar-brand" href="#">Start Bootstrap</a>
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarResponsive" aria-controls="navbarResponsive" aria-expanded="false" aria-label="Toggle navigation">
          <span class="navbar-toggler-icon"></span>
        </button>
    <div class="collapse navbar-collapse" id="navbarResponsive">
      <ul class="navbar-nav ml-auto">
        <li class="nav-item active">
          <a class="nav-link" href="#">Home
                <span class="sr-only">(current)</span>
              </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">
          <a class="nav-link" href="#">Contact</a>
        </li>
      </ul>
    </div>
  </div>
</nav>

<!-- Page Content -->
<div class="container">
  <div class="card mt-4">
    <div class="card-body">
      <h1>Page Content</h1>
      <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Modi voluptas sit blanditiis. Eveniet deleniti ipsam nobis nulla illo excepturi iure sint fugit, laboriosam quidem, fuga assumenda neque alias suscipit inventore.</p>
    </div>
  </div>
</div>
body {
  background: url('http://placehold.it/1920x1080') no-repeat center center fixed;
  -webkit-background-size: cover;
  -moz-background-size: cover;
  background-size: cover;
  -o-background-size: cover;
}

              

External resources loaded into this fiddle: