.header_nav {
  display: flex;
  justify-content: center;
  align-items: center;
  background: #0a1429;
  width: 100%;
  height: 50px;
}

.header_nav ul {
  display: flex;
  justify-content: center;
  align-items: center;
  width: 100%;
}
.header_nav ul li {
  list-style: none;
  margin: 0 1rem;
  flex: 1;
  text-align: center;
}
.header_nav ul li a {
  color: #00d4c1;
  text-decoration: none;
  font-size: 1.2rem;
}

.header_nav ul li a.on {
  color: #fff;
}

@media screen and (max-width: 768px) {
  .header_nav {
    height: auto;
  }
  .header_nav ul {
    flex-wrap: wrap;
  }
  .header_nav ul li {
    flex: 0 0 50%;
    margin: 0 0 1rem 0;
  }
}