Dark about:home and about:newtab for Firefox

userContent.css

/* Copy to %appdata%\Mozilla\Firefox\Profiles\<profile>\chrome */

@-moz-document url("about:newtab"), url("about:home")
{
  body{
    background: #252525 !important;
    color: #ddd !important;
  }

  #search-wrapper,
  .search-wrapper .search-label,
  .search-wrapper .search-button,
  .card-outer .card-context-icon,
  .card-outer .context-menu-button,
  .top-site-outer .context-menu-button,
  .prefs-pane-button button {
    filter: invert(1) !important;
  }

  .collapsible-section.active {
    background: rgba(11, 11, 11, 0.6) !important;
  }

  .card-outer,
  .prefs-pane .sidebar,
  .prefs-pane .actions {
    background: #151515 !important;
    color: #ddd !important;
  }

  .prefs-pane .sidebar,
  .prefs-pane .actions {
    border-left: 1px solid #777 !important;
  }

  .card-outer:-moz-any(:hover, :focus, .active):not(.placeholder),
  .top-sites-list:not(.dnd-active) .top-site-outer:-moz-any(.active, :focus, :hover) .tile {
    box-shadow: 0 0 0 5px #777 !important;
  }
}