nav ul {
  display: flex;
  justify-content: space-around;
  list-style-type: none;
  padding: 0;
  margin: 0;
  flex-wrap: wrap;
}

nav ul li {
  flex-grow: 1;
  flex: 1 1 auto;
  text-align: center;
}

nav ul li a,
.selected-item {
  display: block;
  text-align: center;
  padding: 10px;
  text-decoration: none;
  box-sizing: border-box;
  border: 2px solid #777;
  color: #fff;
}

nav ul li a {
  background-color: #666;
}

.selected-item {
  display: none;
  background-color: #333;
}

.selected-item::before,
.selected-item::after {
  content: "▼";
  position: relative;
  transition: transform 0.3s ease, opacity 0.3s ease;
}

.selected-item::before {
  left: -5px;
}

.selected-item::after {
  right: -5px;
}

.selected-item.expanded::before,
.selected-item.expanded::after {
  content: "▲";
  transform: rotate(180deg);
}

nav ul li a:hover {
  background-color: #444;
}

nav ul li a.active {
  background-color: #444;
  font-weight: bold;
  box-shadow: 0 0 10px #000;
}