.alinatest nav.advancedmegamenu.mainnav {
  position: relative; /* needed for ::before positioning */
  background: none;   /* clear any existing background */
  width: 100%;
  z-index: 1;
}

.alinatest nav.advancedmegamenu.mainnav::before {
  content: "";
  position: absolute;
  inset: 0;           /* top:0; left:0; right:0; bottom:0 */
  background: black !important;
  z-index: -1;        /* behind nav content */
}