bootstrap/docs/assets/scss/_anchor.scss

.anchorjs-link {
  color: inherit;
}

@media (max-width: 480px) {
  .anchorjs-link {
    display: none;
  }
}

*:hover > .anchorjs-link {
  opacity: .75;
  transition: color .16s linear;
}

*:hover > .anchorjs-link:hover,
.anchorjs-link:focus {
  text-decoration: none;
  opacity: 1;
}