Simple breakpoint mixin

media.scss

// breakpoints from bootstrap 4
$grid-breakpoints: (
  xs: 0,
  sm: 576px,
  md: 768px,
  lg: 992px,
  xl: 1200px
) !default;

@mixin breakpoint($point) {
  @each $key, $value in $grid-breakpoints {
    @if $key == $point {
      @media (min-width: $value) { 
        @content; 
      }
    }
  }
}

// usage

.some-text {
  font-size: 18px;
  
  @include breakpoint(sm) {
    font-size: 14px;
  }
}