JSFiddle - React, Tailwind, and code Playground

HTML

<header>
    
    <nav id="access">test</nav>

</header>

CSS

header{
    margin: 50px auto;
    width:400px;
}


#access{
	background-color: #3C5468;
	position: relative;
	margin-left: -15px;
	margin-right: -15px;
}

#access:before, #access:after{
   display: block;
   width: 10px;
   height: 0px;
   position: absolute;
   bottom: -15px;
   content: "";
   border-bottom: 15px solid transparent;
}

#access:before{
   left: -10px;
   border-right: 15px solid rgb(27, 48, 65);
}

#access:after{
   right: -10px;
   border-left: 15px solid rgb(27,48,65);
}