BookStack/resources/sass/_mixins.scss

// Responsive breakpoint control
@mixin smaller-than($size) {
    @media screen and (max-width: $size) { @content; }
}
@mixin larger-than($size) {
    @media screen and (min-width: $size) { @content; }
}
@mixin between($min, $max) {
  @media screen and (min-width: $min) and (max-width: $max) { @content; }
}