from subtle's description: "This layout was designed to have a background that is subtle, seamless pattern or a solid color."
css only supports a pattern or solid color, it was not designed for a full image bg

you can write your own css to edit it any way you wish but as outlined in my rules (https://toyhou.se/~bulletins/8675.css-html-shop-rules) the buyer is responsible for all edits they wish to make