Add the following snippet to the existing userstyle

main,
  #app .grid.w-full,
  #app > div > header + div {
    background-image: url('https://staging.cohostcdn.org/attachment/e2050e3d-e687-4bb8-ad27-25762aa10997/CleanShot%202022-08-05%20at%2009.37.53.png');
    background-attachment: fixed;
    background-repeat: repeat;
  }
  @media (prefers-color-scheme: dark) {
    main,
    #app .grid.w-full,
    #app > div > header + div {
      background-image: url('https://staging.cohostcdn.org/attachment/ddd0e75a-efe2-4ec6-a835-e0155890bf69/CleanShot%202022-08-05%20at%2009.37.46.jpg');
    }
  }