bootstrap-scss-breakpoint-mixin-mobile-first.scss

$screen-sm-min: 768px;
$screen-xs-max: ($screen-sm-min - 1);

$screen-md-min: 992px;
$screen-sm-max: ($screen-md-min - 1);

$screen-lg-min: 1200px;
$screen-md-max: ($screen-lg-min - 1);

@mixin breakpoint($class) {
  @if $class == xs {
    @media (max-width: $screen-md-max) { @content; }
  }
  @else if $class == sm {
    @media (min-width: $screen-md-min) { @content; }
  }
  @else if $class == lg {
    @media (min-width: $screen-lg-min) { @content; }
  }
  @else {
    @warn "Breakpoint mixin supports: xs, sm, lg";
  }
}