style.css

tabbed-custom-element::part(tab) {
  color: #0c0c0dcc;
  border-bottom: transparent solid 2px;
}

tabbed-custom-element::part(tab):hover {
  background-color: #0c0c0d19;
  border-color: #0c0c0d33;
}

tabbed-custom-element::part(tab):hover:active {
  background-color: #0c0c0d33;
}

tabbed-custom-element::part(tab):focus {
  box-shadow:
    0 0 0 1px #0a84ff inset,
    0 0 0 1px #0a84ff,
    0 0 0 4px rgba(10, 132, 255, 0.3);
}

tabbed-custom-element::part(active) {
  color: #0060df;
  border-color: #0a84ff !important;
}