TRaSH-Guides/docs/stylesheets/github-permalink-style.css

.headerlink {
  --permalink-size: 20px; /* for font-relative sizes, 0.6em is a good choice */
  --permalink-spacing: 4px;

  width: calc(var(--permalink-size) + var(--permalink-spacing));
  height: var(--permalink-size);
  vertical-align: middle;
  background-color: var(--md-default-fg-color--lighter);
  background-size: var(--permalink-size);
  mask-size: var(--permalink-size);
  -webkit-mask-size: var(--permalink-size);
  mask-repeat: no-repeat;
  -webkit-mask-repeat: no-repeat;
  visibility: visible;
  mask-image: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" version="1.1" width="16" height="16" aria-hidden="true"><path fill-rule="evenodd" d="M7.775 3.275a.75.75 0 001.06 1.06l1.25-1.25a2 2 0 112.83 2.83l-2.5 2.5a2 2 0 01-2.83 0 .75.75 0 00-1.06 1.06 3.5 3.5 0 004.95 0l2.5-2.5a3.5 3.5 0 00-4.95-4.95l-1.25 1.25zm-4.69 9.64a2 2 0 010-2.83l2.5-2.5a2 2 0 012.83 0 .75.75 0 001.06-1.06 3.5 3.5 0 00-4.95 0l-2.5 2.5a3.5 3.5 0 004.95 4.95l1.25-1.25a.75.75 0 00-1.06-1.06l-1.25 1.25a2 2 0 01-2.83 0z"></path></svg>');
  -webkit-mask-image: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" version="1.1" width="16" height="16" aria-hidden="true"><path fill-rule="evenodd" d="M7.775 3.275a.75.75 0 001.06 1.06l1.25-1.25a2 2 0 112.83 2.83l-2.5 2.5a2 2 0 01-2.83 0 .75.75 0 00-1.06 1.06 3.5 3.5 0 004.95 0l2.5-2.5a3.5 3.5 0 00-4.95-4.95l-1.25 1.25zm-4.69 9.64a2 2 0 010-2.83l2.5-2.5a2 2 0 012.83 0 .75.75 0 001.06-1.06 3.5 3.5 0 00-4.95 0l-2.5 2.5a3.5 3.5 0 004.95 4.95l1.25-1.25a.75.75 0 00-1.06-1.06l-1.25 1.25a2 2 0 01-2.83 0z"></path></svg>');
}

[id]:target .headerlink {
  background-color: var(--md-typeset-a-color);
}

.headerlink:hover {
  background-color: var(--md-accent-fg-color) !important;
}

@media screen and (min-width: 76.25em) {
  h1,
  h2,
  h3,
  h4,
  h5,
  h6 {
    display: flex;
    align-items: center;
    flex-direction: row;
  }

  .headerlink {
    order: -1;
    margin-left: calc(
      var(--permalink-size) * -1 - var(--permalink-spacing)
    ) !important;
  }
}