FreeTube/src/renderer/components/ft-community-post/slider-style.css

.tns-nav .tns-nav-active {
  background-color: #999;
}

.tns-nav button {
  background-color: #ddd;
  border-radius: 50%;
  block-size: 1.5em;
  padding: 0;
  inline-size: 1.5em;
}