* {
    box-sizing: border-box;
}

    body {
        display: grid;
        grid-gap: 1em;
    }

    #header {
        margin: -0.7em; /* Fill entire length of screen */
        /* Define layout for the children elements */
        display: grid;
    }

    #header #header-nav {
        /* Place it in the parent element */
        grid-column-start: 1;
        grid-column-end: 2;
        display: flex;
    }