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

// @mixin display-flex {
//   display: -webkit-box;
//   display: -webkit-flex;
//   display: -ms-flexbox;
//   display: flex;
// }

@mixin flex($string) {
  @if $string == 'auto' {
    $string: 1
  }

  -webkit-box-flex: $string;
  -webkit-flex: $string;
  -ms-flex: $string;
  flex: $string;
}

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

// @mixin display-flex {
//   display: -webkit-box;
//   display: -webkit-flex;
//   display: -ms-flexbox;
//   display: flex;
// }

@mixin flex($string) {
  @if $string == 'auto' {
    $string: 1
  }

  -webkit-box-flex: $string;
  -webkit-flex: $string;
  -ms-flex: $string;
  flex: $string;
}

.transparent {
  opacity: 0;
}

.opaque {
  opacity: 1;
}

Add some opacity manipulating utility classes
Add some opacity manipulating utility classes

Transparent and opaque do what you think.
