body {
    background: linear-gradient(100deg, #fff 50%, #8B0000 50.2%);
    height: 100vh;
    margin: 0 1em;
    display: grid;
    grid: 1fr / auto-flow;
    grid-column-gap: 3rem;
    place-items: center;
}

aside {
    display: none;
}

main {
    width: 400px;
}

@media(min-width: 1000px) {
    aside {
        display: block;
    }
}