JSFiddle - React, Tailwind, and code Playground

HTML

<div class="header"><h1>This is a header</h1></div>
<nav class="navbar"></nav>

CSS

.header{
      position:relative;
      color:white;
      background-color:#2B3A48;
      text-align:center;
}
.navbar {
      background: #272220;
      height:20px;
}
.header:after {
      content: "";
      display: block;
      position: absolute;
      bottom: -10px;
      left:0;
      right:0;
      height: 10px;
      /* TODO Add browser prefixes */
      background:
        linear-gradient(
          45deg, transparent 33.333%,
          #2B3A48 33.333%, #2B3A48 66.667%,
          transparent 66.667%
        ),
        linear-gradient(
          -45deg, transparent 33.333%,
          #2B3A48 33.333%, #2B3A48 66.667%,
          transparent 66.667%
        );
        background-size: 8px 20px;
        background-position: 0 -10px;
    }