[css] Sass - Converting Hex to RGBa for background opacity

you can try this solution, is the best... url(github)

// Transparent Background
// From: http://stackoverflow.com/questions/6902944/sass-mixin-for-background-transparency-back-to-ie8

// Extend this class to save bytes
.transparent-background {
  background-color: transparent;
  zoom: 1;
}

// The mixin
@mixin transparent($color, $alpha) {
  $rgba: rgba($color, $alpha);
  $ie-hex-str: ie-hex-str($rgba);
  @extend .transparent-background;
  background-color: $rgba;
  filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#{$ie-hex-str},endColorstr=#{$ie-hex-str});
}

// Loop through opacities from 90 to 10 on an alpha scale
@mixin transparent-shades($name, $color) {
  @each $alpha in 90, 80, 70, 60, 50, 40, 30, 20, 10 {
    .#{$name}-#{$alpha} {
      @include transparent($color, $alpha / 100);
    }
  }
}

// Generate semi-transparent backgrounds for the colors we want
@include transparent-shades('dark', #000000);
@include transparent-shades('light', #ffffff);