$desktop-min-breakpoint: 640px;
$mobile-max-breakpoint: $desktop-min-breakpoint - 1px;

@mixin mobile {
  @media only screen and (max-width: $mobile-max-breakpoint) {
    @content
  }
}

@mixin desktop {
  @media only screen and (min-width: 640px) {
    @content
  }
}