@mixin opacity($value){
  -webkit-opacity: $value;
  -moz-opacity: $value;
  opacity: $value;
}

@mixin transition($properties){
  -webkit-transition: $properties;
  -moz-transition: $properties;
  -ms-transition: $properties;
  -o-transition: $properties;
  transition: $properties;
}

@mixin cover(){
  position: fixed; /* Stay in place */
  z-index: 1; /* Sit on top */
  left: 0;
  top: 0;
  width: 100%; /* Full width */
  height: 100%; /* Full height */
  overflow: hidden;
  background-color: rgb(0,0,0); /* Fallback color */
}