when using this battle tested (and free!) library - you could display icons by using "fa" prefixed classes in your HTML:
<header class="topbar">
<a id="sidebar-toggle" class="sidebar-toggle" href="javascript:void(0);">
<i class="fa fa-bars"></i>
</a>
<div class="input-wrapper">
<input type="text"><i class="fa fa-search"></i>
</div>
<nav class="right-nav">
<a href="javascript:void(0);">
<i class="fa fa-bell"></i>
</a>
<a href="javascript:void(0);">
<i class="fa fa-comment"></i>
</a>
</nav>
</header>
Instead of install the library files locally I prefer to use online CDN. That way it can easily included in project:https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.8.1/css/all.css