@define-mixin mask $bg{
  background-color: $bg;
  position: fixed;
  z-index: 999;
  bottom: 0;
  right: 0;
  left: 0;
  top: 0;
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-pack: center;
  -webkit-justify-content: center;
  -ms-flex-pack: center;
  justify-content: center;
  -webkit-box-align: center;
  -webkit-align-items: center;
  -ms-flex-align: center;
  align-items: center;
}