@media check

  @media {
    background-color: #f7f7f7;
    /*
    combo box popup window rendered outside cabinetUI
    */
    gwt-MenuItem {
      font-size: 16px;
      color: red;
    }
  }

  /*[width-range~="768px-991px"]*/
  @media (max-width:991px) {
    background-color: white;

    gwt-MenuItem {
      font-size: 14px;
      color: yellow;
    }
  }

  /*[width-range~="0-767px"]*/
  @media (max-width:767px) {
    background-color: grey;

    gwt-MenuItem {
      font-size: 12px;
      color: green;
    }
  }
/*
  .gwt-MenuItem {
    @media {
      font-size: 24px;
    }
    @media (max-width:991px) {
      font-size: 22px;
    }
    @media (max-width:767px) {
      font-size: 20px;
    }
  }*/


$myVar1: 22px;
$myVar2: 10px;

@media (max-width:991px) {
  .mytheme {
    .gwt-MenuItem {
      font-size: $myVar1;
    }
  }
}
@media (max-width:767px) {
  .mytheme {
    .gwt-MenuItem {
      font-size: $myVar2;
    }
  }
}