.header-nav-wrap{

  z-index: 1000;
  width: 100%;
  height: 100vh;
  position: fixed;
display: none;
background-color: gray;
}
.nav_menu {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translateY(-50%) translateX(-50%);
    -webkit- transform: translateY(-50%) translateX(-50%);
    margin: auto;
    color: white;
}


@media screen and (max-width: 750px) {

.nav_menu li{
line-height: 1em;
	font-size:50px;
}
.header-nav-wrap{
  z-index: 1000;
  width: 100%;
  height: 100vh;
  position: fixed;
display: none;
background-color: gray;
}
  }