body {
    color: darkslategrey;
    background-color: floralwhite;
    font-size: 18px;
    line-height: 1.8;
    margin: auto;
    padding: 0 16px;
    max-width: 700px;
}
aside {
    font-size: 16px;
    position: absolute;
    background-color: white;
    top: 0; left: 0; right: 0;
    text-align: center;
}
a {
    color: inherit;
}

@media (prefers-color-scheme: dark) {
    body {
        color: floralwhite;
        background-color: #0f1a1a;
    }
    aside {
        background-color: black;
    }
}