@mixin container($width: $container-width) {
  width: $width;
  margin: 0 auto;
  padding: 2em 0 5em 0;

  @media only screen and (max-width: 640px) {
    width: auto;
    margin: 0 0.6em;
    padding: 0.6em 0;
  }
}

@mixin row {
  @include clearfix;
}

@mixin columns($num, $margin: 1.25%, $padding: 0%) {
  $single-width: (100%) / 12;

  width: $num * $single-width - $margin - 2 * $padding;
  margin-right: $margin;
  padding: $padding;
  float: left;
}

@mixin container($width: $container-width) {
  width: $width;
  margin: 0 auto;
  padding: 2em 0 5em 0;

  @media only screen and (max-width: 640px) {
    width: auto;
    margin: 0 0.6em;
    padding: 0.6em 0;
  }
}

@mixin row {
  @include clearfix;
  margin-bottom: 5px;
}

@mixin columns($num, $margin: 1.25%, $padding: 0%) {
  $single-width: (100%) / 12;

  width: $num * $single-width - $margin - 2 * $padding;
  margin-right: $margin;
  padding: $padding;
  float: left;
}

Add bottom margin to grid rows.
Add bottom margin to grid rows.