cdk-listbox-horizontal-example.css

.example-listbox {
  display: flex;
  width: 250px;
  padding: 0;
}

.example-option {
  display: flex;
  flex: 1;
  align-items: center;
  justify-content: center;
  position: relative;
  list-style: none;
  padding: 12px;
  border: solid black;
  border-width: 1px 1px 1px 0;
}

.example-option:first-child {
  border-left-width: 1px;
}

.example-option[aria-selected='true']::before {
  content: '';
  position: absolute;
  border: 2px solid black;
  top: 3px;
  bottom: 3px;
  left: 3px;
  right: 3px;
}

.example-option:focus {
  background: rgba(0, 0, 0, 0.2);
}