netbox-community-netbox/netbox/project-static/styles/transitional/_tabs.scss

// In-page content tabs
.page-tabs {
  // Set the border on page-tabs to ensure it expands the full page width
  border-bottom: 1px solid $card-border-color;

  .nav-tabs {
    position: relative;
    border: none;

    .nav-link {
      &.active,
      &:active,
      &:hover {
        border-color: $card-border-color;
        border-bottom-color: transparent;
      }

      &.active {
        color: $headings-color;
        background: var(--#{$prefix}bg-surface-secondary);
        border-bottom-color: transparent;
      }
    }
  }
}