autonomic-block-theme/block-styles.css

/* For the "Tabbed Heading" block style */
.is-style-tabbed-heading {
  display: flex;
  height: 36px;
  align-items: center;
  border-top: 1px solid;
}

.is-style-tabbed-heading::before {
  content: "";
  border-bottom: 1px solid;
  border-right: 1px solid;
  width: 80px;
  margin-right: 8px;
  height: 36px;
  border-top: 2px solid var(--wp--preset--color--background);
}

.is-style-tabbed-heading::after {
  content: "";
  border-bottom: 1px solid;
  flex-grow: 1;
  margin-left: 8px;
  height: 36px;
  border-left: 1px solid;
  border-top: 2px solid var(--wp--preset--color--background);
}

/* For the "Markdown-style Heading" block style */
.is-style-markdown-heading::before {
  content: "##";
  width: 4ch;
  margin-right: 0;
  font-family: monospace;
  margin-inline-end: .3em;
  letter-spacing: 2px;
}

@media screen and (max-width: 782px) {
  .is-style-markdown-heading::before {
    margin-inline-start: 1em;
  }
}

@media screen and (min-width: 782px) {
  .is-style-markdown-heading {
    margin-top: 4em;
    font-size: var(--wp--preset--font-size--medium);
  }
}