@function background_color($value)
  @if color($value)
    @return true
  @else
    @return null

@function background_position($value)
  @if list($value)
    @if str-slice(nth($value, 1), 1, 1) != '/'
      @if position(nth($value, 1)) or position(nth($value, 2))
        @return true
      @else if number(nth($value, 1)) or number(nth($value, 2))
        @return true
      @else
        @return null
    @else
      @return null
  @else
    @return null

@function background_size($value)
  @if string($value)
    @if $value == 'auto' or $value == 'contain' or $value == 'cover'
      @return true
    @else if str-slice($value, 2) == 'auto' or $value == 'contain' or $value == 'cover'
      @return $value
    @else
      @return null
  @else if list($value)
    @if str-slice(nth($value, 1), 1, 1) == '/'
      @if number(nth($value, 1)) or number(nth($value, 2))
        @return true
      @else
        @return null
  @else
    @return null

@function background_repeat($value)
  @if $value == 'no-repeat' or $value == 'repeat' or $value == 'repeat-x' or $value == 'repeat-y'
    @return true
  @else
    @return null

@function background_attachment($value)
  @if $value == 'fixed' or $value == 'scroll'
    @return true
  @else
    @return null

@function background_image($value)
  @if image($value)
    @return true
  @else
    @return null