simple modification to alter the discourse dark theme

discourse-dark.css

/* common */
body {
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-position: center;
}
#main-outlet{
    background-color: rgba(30, 30, 30, 0.9);
}

/* desktop */
body{
    background-image: url('url_to_image');
}
/* mobile */
body{
    background-image: url('url_to_image');
}