$breakpoints-max: (
  small: 600px,
  medium: 960px,
  large: 1280px
);

$breakpoints-min: (
  small: 601px,
  medium: 961px,
  large: 1281px
);

@mixin media-max($screen-size) {
  @if map-has-key($breakpoints-max, $screen-size) {
    @media (max-width: map-get($breakpoints-max, $screen-size)) {
      @content;
    }
  } @else {
    // Debugging
    @warn "'#{$screen-size}' has not been declared as a breakpoint."
  }
}

@mixin media-min($screen-size) {
  @if map-has-key($breakpoints-min, $screen-size) {
    @media (min-width: map-get($breakpoints-min, $screen-size)) {
      @content;
    }
  } @else {
    // Debugging
    @warn "'#{$screen-size}' has not been declared as a breakpoint."
  }
}