Firefoxflat_owl.css

/* userChrome.css */
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

:root:-moz-lwtheme-brighttext {
  --chrome-background-color: #555555 !important;
  --chrome-color: #999999 !important;
  --chrome-secondary-background-color: #444444 !important;
  --toolbox-border-bottom-color: #424242 !important;
  --url-and-searchbar-border-color: #424242 !important;
  --focus-ring-box-shadow: #333333 !important;
}

#urlbar[focused="true"], .searchbar-textbox[focused="true"] {
  border-color: #525252 !important;
}

.browserContainer {
  background-color: #555555 !important;
}

:root:-moz-lwtheme {
  --tab-line-color: none !important;
}

/* userContent.css */
@-moz-document url(about:newtab) {
    .activity-stream {
        background-color: #444444 !important;
    }
    .top-sites-list .top-site-outer > a {
        color: #737373 !important;
    }
    .top-sites-list .top-site-outer .title.pinned span {
        font-size: smaller !important;
    }
    .tile {
      filter: grayscale(42%) !important;
      opacity: .9 !important;
    }
}

/* userChrome.css : to make normal density theme bit compact */
:root:not([uidensity=compact]) #back-button {
  padding-top: 0 !important;
  padding-bottom: 0 !important;
}
:root:not([uidensity=compact]) #back-button > .toolbarbutton-icon {
  background-color: unset !important;
  border: none !important;
}