div.container {
  display: flex;
  width: 600px;
  algin-items: center;
  justify-content: flex-start;
}

.menu {
  list-style-type: none;
  display: flex;
  width: 600px;
  margin: 0 auto;
}

.menu li {
  border: 1px solid black;
  width: 100px;
  text-align: center;
  display: inline-block;

}

a:hover {
  color:green;
  background-color: light-gray;
}