sass-site/source/assets/css/_breakpoints.scss

$sl-breakpoint--small:   480px  !default;
$sl-breakpoint--medium:  640px  !default;
$sl-breakpoint--large:   960px  !default;
$sl-breakpoint--x-large: 1280px !default;

$sl-breakpoints: (
  none:    0, // for mixins to span across breakpoints and without one
  small:   $sl-breakpoint--small,
  medium:  $sl-breakpoint--medium,
  large:   $sl-breakpoint--large,
  x-large: $sl-breakpoint--x-large,
);

@mixin sl-breakpoint($breakpoint) {

  @media only screen and (min-width: #{sl-px-to-em($breakpoint)}) {
    @content;
  }
}

@mixin sl-breakpoint-max($breakpoint) {

  @media only screen and (max-width: #{sl-px-to-em($breakpoint)}) {
    @content;
  }
}

@mixin sl-breakpoint--small {

  @include sl-breakpoint($sl-breakpoint--small) {
    @content;
  }
}

@mixin sl-breakpoint--small-max {

  @include sl-breakpoint-max($sl-breakpoint--small) {
    @content;
  }
}

@mixin sl-breakpoint--medium {

  @include sl-breakpoint($sl-breakpoint--medium) {
    @content;
  }
}

@mixin sl-breakpoint--medium-max {

  @include sl-breakpoint-max($sl-breakpoint--medium) {
    @content;
  }
}

@mixin sl-breakpoint--large {

  @include sl-breakpoint($sl-breakpoint--large) {
    @content;
  }
}

@mixin sl-breakpoint--large-max {

  @include sl-breakpoint-max($sl-breakpoint--large) {
    @content;
  }
}

@mixin sl-breakpoint--x-large {

  @include sl-breakpoint($sl-breakpoint--x-large) {
    @content;
  }
}

@mixin sl-breakpoint--x-large-max {

  @include sl-breakpoint-max($sl-breakpoint--x-large) {
    @content;
  }
}

@mixin sl-breakpoint-set($breakpoint,
                         $size) {

  @if $breakpoint == none {
    @content;
  }

  @else {

    @include sl-breakpoint($size) {
      @content;
    }
  }
}