body{
    background-image: url(img/poly.jpg);
    height: 100%;
    width: 100%;
    background-repeat: no-repeat;
    background-size: cover;
}
li{
    list-style-type: none;
}
.logo_center {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
  }