Responsive design usando sass

     $sm: 10px;
$md: 768px;
$lg: 992px;

@mixin mobile {
  @media (max-width: $md) {
    @content;
  }
}

@mixin tablet {
  @media (min-width: $md) {
    @content;
  }
}

@mixin desktop {
  @media (min-width: $lg) {
    @content;
  }
}