:root {
    --header-height: 6em; /* or whatever the height of your header is */
}

body {
    display: flex;
    flex-direction: column;
    min-height: 100vh;
}

body > header {
    position: fixed;
    width: 100%;
    height: var(--header-height);
}

header div{
    display: flex;
    flex-direction: row-reverse;
    justify-content: start;
    background: #999;
    height: 100%;
}

header div img {
    padding: .5em;
}

header nav ul {
    display: none;
}

main {
    margin: var(--header-height) auto auto;
    padding: 1em;
    flex: 1 0 auto;
}

main article {
    border: black inset 1px;
    padding: 1em;
    margin: .2em;
}

main aside {
    display: none;
}

body > footer {
    background: #999;
    padding: 1em;
}

@media (min-width: 1024px) {

    header nav ul {
        display: flex;
        flex-direction: row;
        justify-content: space-evenly;
        background: #666;
        list-style-type: none;
        margin: auto;
    }

    header nav ul a {
        text-decoration: none;
        color: white;
    }

    main {
        width: 960px;
    }

    main #content {
        display: flex;
        flex-flow: row wrap;
        align-content: space-between;
    }

    main article {
        width: 48%;
    }

    main aside {
        display: block;
        float: right;
        width: 30%;
        background: #ccc;
        /*border: black solid 1px;*/
        margin: .2em;
    }

    main aside > section {
        border: #999 inset 1px;
    }

    main aside h2 {
        color: #fff;
        background: #666;
        padding: .5em;
        margin: 0;
    }

    main aside section p {
        padding: 1em;
    }

    main aside footer {
        color: #fff;
        background: #666;
        padding: .5em;
    }

}