border($width = 1px, $style = solid, $color = $gray100, $radius = null)
  border-color: $color
  border-style: $style
  border-width: $width
  if $radius
    border-radius($radius)

border-radius($radius = $default-radius)
  border-radius: $radius
  -moz-border-radius: $radius
  -webkit-border-radius: $radius