JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <div id="background"></div>
    <header>
        <ul>
            <li>Button 1</li>
            <li>Button 2</li>
        </ul>
    </header>
</body>

CSS

* { margin:0; padding:0; }
html { width:100%; height:100%; }
body { width:100%; height:100%; }
#background { position:absolute; width:100%; height:200px; background:#989898; z-index:-1; }
header {width:100%; height:100px; margin-top:50px; background:rgba(0,0,0,0.4);}