@mixin mask-image($img, $repeat: no-repeat){
   @include experimental(mask-image, image-url($img), -webkit, -moz, -o, -ms);
   @include experimental(mask-repeat, $repeat, -webkit, -moz, -o, -ms);
   width: image-width($img);
   height: image-height($img);
 }
 

 @mixin shadow-box($border: #fff .5em solid, $shadow: rgba(#000, .15) 0 1px 4px, $border-radius: .3em) {
   @include border-radius($border-radius);
   @include box-shadow($shadow);
   @include box-sizing(border-box);
   border: $border;
 }
 

 @mixin selection($bg, $color: inherit, $text-shadow: none){
   * {
     &::-moz-selection { background: $bg; color: $color; text-shadow: $text-shadow; }
     &::-webkit-selection { background: $bg; color: $color; text-shadow: $text-shadow; }
     &::selection { background: $bg; color: $color; text-shadow: $text-shadow; }
   }
 }
 
 @function text-color($color, $dark: dark, $light: light){
   $text-color: ( (red($color)*299) + (green($color)*587) + (blue($color)*114) ) / 1000;
   $text-color: if($text-color >= 150, $dark, $light);
   @return $text-color;
 }