Exclude Mobile CSS into separate stylesheet

_main.scss

@import "mixins";

body {
    font-family: 'Open Sans', 'Helvetica Neue', Helvetica, Arial, sans-serif;
}

.container {
    width: 960px;
    margin: 0 auto;

    @include respond-to(small) {
        width: 100%;
        margin: 0;
    }

    @include respond-to(large) {
        background: url(fancyLargeImage.jpg);
    }
}

_mixins.scss

$isMobile: false !default;

@mixin respond-to($media) {
  @if $media == small and $isMobile {
    @content;
  }
  @else if $media == small {
    @media only screen and (max-width: 479px) { @content; }
  }
  @else if $media == medium and not $isMobile {
    @media only screen and (min-width: 480px) and (max-width: 767px) { @content; }
  }
  @else if $media == large and not $isMobile {
    @media only screen and (min-width: 768px) and (max-width: 959px) { @content; }
  }
}

mobile.scss

$isMobile: true;
@import "main";

styles.scss

@import "main";