* {
  color: #fff;
}

body {
  background: url(images/bg.jfif) no-repeat;
  background-size: cover;
  height: 100vh;
}

ul {
  padding: 0;
  margin: 0 auto;
  list-style: none;
}

#all {
    margin-left: 150px;
}
ul li {
  float: left;
  width: 200px;
  height: 50px;
  background: #000;
  opacity: 0.8;
  line-height: 50px;
  text-align: center;
  font-size: 20px;
  margin-right: 2px;
}

ul li a {
    display: block;
    text-decoration: none;
    color: white;
}

ul li a:hover {
    background: green;
}

ul li ul li {
    display: none;
}

ul li:hover ul li {
    display: block;
}