IE gradient mixin with SCSS/SASS

mixins.css.scss

@mixin gradient($first, $second) {
  background-color: $second;
  -ms-filter: "progid:DXImageTransform.Microsoft.gradient(GradientType=0, startColorstr='#{ie-hex-str($first)}', endColorstr='#{ie-hex-str($second)}');";
  filter: progid:DXImageTransform.Microsoft.gradient(GradientType=0, startColorstr='#{ie-hex-str($first)}', endColorstr='#{ie-hex-str($second)}');;
  zoom: 1;
  background: -webkit-gradient(linear, left top, left bottom, from($first), to($second));
  background: -moz-linear-gradient(top,  $first,  $second);
}

sass_helpers.rb

require 'sass'

module Sass::Script::Functions
  # LARS: Snatched from compass - 2011-11-29 - used for gradients in IE6-9
  # returns an IE hex string for a color with an alpha channel
  # suitable for passing to IE filters.
  def ie_hex_str(color)
    assert_type color, :Color
    alpha = (color.alpha * 255).round
    alphastr = alpha.to_s(16).rjust(2, '0')
    Sass::Script::String.new("##{alphastr}#{color.send(:hex_str)[1..-1]}".upcase)
  end      
  declare :ie_hex_str, [:color]
end

Answering my own question: I need to "@import" the mixin scss file in active_admin.css.scss

BTW setting "alphastr" to "00" works better for activeadmin buttons (the default is "FF", displaying a white background for rounded buttons).