This CSS & JS library uses Bootstrap 4 + FontAwesome 5 to create a simple admin template with a collapsing & responsive sidebar.
Include bsadmin.css & bsadmin.js in your code, and see the example below.
Make sure to include all other dependencies shown in the example.
<!doctype html>
<html lang="en">
<head>
<title>Hello, world!</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
<link rel="stylesheet" href="css/bootstrap.min.css">
<link rel="stylesheet" href="css/fontawesome.min.css">
<link rel="stylesheet" href="css/bsadmin.css">
</head>
<body>
<nav class="navbar navbar-expand navbar-dark bg-primary">
<a class="sidebar-toggle text-light mr-3"><i class="fa fa-bars"></i></a>
<a class="navbar-brand" href="#"><i class="fa fa-code-branch"></i> Navbar</a>
<div class="navbar-collapse collapse">
<ul class="navbar-nav ml-auto">
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdownMenuLink" data-toggle="dropdown">
<i class="fa fa-user"></i> Username
</a>
<div class="dropdown-menu dropdown-menu-right" aria-labelledby="navbarDropdownMenuLink">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another</a>
</div>
</li>
</ul>
</div>
</nav>
<div class="d-flex align-items-stretch">
<div class="sidebar bg-dark">
<ul class="list-unstyled">
<li><a href="#"><i class="fa fa-fw fa-link"></i> Sidebar Link</a></li>
<li><a href="#"><i class="fa fa-fw fa-link"></i> Sidebar Link</a></li>
<li>
<a href="#submenu1" data-toggle="collapse"><i class="fa fa-fw fa-address-card"></i> Dropdown Link</a>
<ul id="submenu1" class="list-unstyled collapse">
<li><a href="#">Submenu Item</a></li>
<li><a href="#">Submenu Item</a></li>
<li><a href="#">Submenu Item</a></li>
</ul>
</li>
<li>
<a href="#submenu2" data-toggle="collapse"><i class="fa fa-fw fa-address-card"></i> Dropdown Link 2</a>
<ul id="submenu2" class="list-unstyled collapse">
<li><a href="#">Submenu Item</a></li>
<li><a href="#">Submenu Item</a></li>
<li><a href="#">Submenu Item</a></li>
</ul>
</li>
<li><a href="#"><i class="fa fa-fw fa-angle-right"></i> Sidebar Link</a></li>
<li><a href="#"><i class="fa fa-fw fa-link"></i> Sidebar Link</a></li>
</ul>
</div>
<div class="content p-4">
<h1 class="display-5 mb-4">Hello, world!</h1>
<p>Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Vestibulum tortor quam, feugiat vitae, ultricies eget, tempor sit amet, ante. Donec eu libero sit amet quam egestas semper. Aenean
ultricies mi vitae est. Mauris placerat eleifend leo. Quisque sit amet est et sapien ullamcorper pharetra. Vestibulum erat wisi, condimentum sed, commodo vitae, ornare sit amet, wisi. Aenean fermentum, elit eget tincidunt
condimentum, eros ipsum rutrum orci, sagittis tempus lacus enim ac dui. Donec non enim in turpis pulvinar facilisis. Ut felis. Praesent dapibus, neque id cursus faucibus, tortor neque egestas augue, eu vulputate magna eros eu erat.
Aliquam erat volutpat. Nam dui mi, tincidunt quis, accumsan porttitor, facilisis luctus, metus</p>
<p>Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Vestibulum tortor quam, feugiat vitae, ultricies eget, tempor sit amet, ante. Donec eu libero sit amet quam egestas semper. Aenean
ultricies mi vitae est. Mauris placerat eleifend leo. Quisque sit amet est et sapien ullamcorper pharetra. Vestibulum erat wisi, condimentum sed, commodo vitae, ornare sit amet, wisi. Aenean fermentum, elit eget tincidunt
condimentum, eros ipsum rutrum orci, sagittis tempus lacus enim ac dui. Donec non enim in turpis pulvinar facilisis. Ut felis. Praesent dapibus, neque id cursus faucibus, tortor neque egestas augue, eu vulputate magna eros eu erat.
Aliquam erat volutpat. Nam dui mi, tincidunt quis, accumsan porttitor, facilisis luctus, metus</p>
</div>
</div>
<script src="js/jquery.min.js"></script>
<script src="js/popper.min.js"></script>
<script src="js/bootstrap.min.js"></script>
<script src="js/bsadmin.js"></script>
</body>
</html>