#menu ul {
    list-style: none;
    margin: 0;
    padding: 0;
    background: blue;
    display: flex;
}

#menu ul li {
      position: relative;
    border-right: 1px solid white;
}

#menu ul li a {
    display: block;
    padding: 10px 20px;
    text-decoration: none;
    color: white;
}

#menu ul li a:hover {
    background: oldlace;
    color: black;
}
#menu ul ul {
    display: none;
    position: absolute;
  top: 100%;              
    background: blue;
    flex-direction: column; 
    min-width: 180px;
}


#menu ul li:hover > ul {
    display: flex; 
}