mobile-config-firefox/chrome/blank_page_background.css

/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/blank_page_background.css made available under Mozilla Public License v. 2.0
See the above repository for updates as well as full license text. */

/* Set blank page background-color */
/* Uses color from theme_colors if available */

#tabbrowser-tabpanels{
  background-color: var(--uc-light-bkgnd-color,rgb(46,54,69)) !important;
}