.tab-arrow::before {
  content: "";
  position: absolute;
  bottom: -6px;
  left: 50%;
  transform: translateX(-50%);
  border-left: 6px solid transparent;
  border-right: 6px solid transparent;
  border-top: 6px solid #0c17a7; /* your active color */
  width: 0;
  height: 0;
  z-index: 10;
}