.dropdown:hover .dropdown-menu{
    display: block;
}

.contenedor-imagen{
    position: relative;
}


.oscureser {
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    height: 100%;
    width: 100%;
    opacity: 0;
    transition: .5s ease;
    background-color: grey;
  }

.contenedor-imagen:hover .oscureser{
    opacity: 0.25;
}

li{
    list-style: none;
    list-style-type: none;
}