<ul>
    <li><a href="/"><span>Main</span></a></li>
    <li><a href="/"><span>Me</span></a></li>
    <li><a href="/"><span>Offer</span></a></li>
    <li><a href="/"><span>Pricing and Promotions</span></a></li>
    <li><a href="/"><span>Classes</span></a></li>
    <li><a href="/"><span>Contact</span></a></li>
  </ul>

ul {
  display: flex;
  padding: 0;
  margin: 0;
  list-style: none;
  background-color: #536453;
  border-right: 0.5em solid #ffcd74;
  border-left: 0.5em solid #ffcd74;
}
li {
  flex: 1 0 auto;
}
ul a {
  display: block;
  padding: 0.25em 1em;
  color: #fff;
  text-decoration: none;
  text-align: center;
  border: 1px solid #fff;
}
a:hover {
  background: red;
}
@media screen and (max-width: 36em){
  ul {
    flex-flow: wrap;
    align-items: stretch;
  }
  ul li {
    flex-basis: 33%;
    display: flex;
    flex-direction: column;
  }
  ul a {
    flex-grow: 1;
  }
}
@media screen and (max-width: 22em){
    ul li {
    flex-basis: 50%;
  }
}
@media screen and (max-width: 15em){
    ul {
    flex-direction: column;
  }
}