JSFiddle - React, Tailwind, and code Playground

HTML

<div id="header"></div>
<div id="container"></div>

CSS

body, html {
    margin: 0;
    height: 100%;
}
#header {
    width: 100%;
    height: 150px;
    background-color: #999999;
}
#container {
    width: 760px;
    height: calc(100% - 150px);
    background-color: #CCCCCC;
    margin: 0 auto;
}