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

@function background_position($value)
  @if list($value) and nth($value, 1) != size
    @return true
  @else
    @return null

@function background_size($value)
  @if string($value)
    @if $value == 'auto' or $value == 'contain' or $value == 'cover'
      @return unquote('/') $value
    @else
      @return null
  @else if list($value)
    @if nth($value, 1) == size
      @return unquote('/') nth($value, 2) nth($value, 3)
    @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

@function background_gradient($value)
  @if string($value)
    @if str-index($value, linear-gradient) or str-index($value, radial-gradient) or str-index($value, repeating-)
      @return true
    @else
      @return null
  @else
    @return null

@function background_clip($value)
  @if string($value)
    @if $value == 'border' or $value == 'padding' or $value == 'content' or $value == 'text'
      @return true
    @else
      @return null
  @else
    @return null