@-moz-document url("about:blank") {
  html {
      background-color: rgb(40,40,40);
  }
}

@-moz-document url(about:newtab) {
    body, #newtab-customize-overlay {
        background-color: rgb(40,40,40) !important;
        color: rgb(166,166,166) !important;
    }
}