userContent.css

/* Background image and color new tab page */

@-moz-document url(about:newtab), url(about:home) {
  .outer-wrapper {
  /*background-color: black !important; /* Background color */
    background-image: url(image.jpg) !important; /* image.jpg in chrome folder */
    background-size: cover !important;
    background-attachment: fixed !important;
    color: grey !important; /* Text color */
  }