border()

@mixin border(
  $size: '',
  $style: solid,
  $color: 'border',
  $radius: 'border-radius',
  $side: 'all'
) {
  // Size.
  $width: 'border-width';
  @if string.length($size) > 0 {
    $width: '#{$width}-#{$size}';
  }

  // Side.
  @if $side == all {
    border: get-border($width, $style, $color);
  } @else if $side == bottom or $side == left or $side == right or $side == top {
    border-#{$side}: get-border($width, $style, $color);
  } @else {
    @error "Unknown side #{$side}.";
  }

  // Radius.
  @if $radius {
    border-radius: get-var($radius);
  }
}