wagtail/client/scss/_tools.breakpoints.scss

$breakpoint-small: $breakpoint-mobile - 0.0625em;
$breakpoint-medium: $breakpoint-mobile;

@mixin small {
    @media only screen and (max-width: $breakpoint-small) {
        @content;
    }
}

@mixin medium {
    @media only screen and (min-width: $breakpoint-medium) {
        @content;
    }
}