metamask-extension/ui/app/css/itcss/components/loading-overlay.scss

.loading-overlay {
  left: 0;
  z-index: 256;
  position: absolute;
  flex-direction: column;
  display: flex;
  justify-content: center;
  align-items: center;
  flex: 1 1 auto;
  width: 100%;
  height: 100%;
  background: rgba(255, 255, 255, .8);

  &__container {
    position: absolute;
    top: 33%;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
  }

  &__message {
    margin-top: 32px;
    font-weight: 400;
    font-size: 20px;
    color: $manatee;
  }
}

.spinner {
  height: 58px;
  width: 58px;
}