Fix defect in DXImageTransform.Microsoft.gradient of generated css

Author: dalelotts

src/css/datetimepicker.css

@@ -55,7 +55,7 @@
     background-repeat: repeat-x;
     border-color: $abdtp-active-border-top-color $abdtp-active-border-top-color $abdtp-active-border-bottom-color;
     color: $abdtp-active-color;
-    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='${abdtp-background-color-start}', endColorstr='${abdtp-background-color-end}', GradientType=0);
+    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#{$abdtp-background-color-start}', endColorstr='#{$abdtp-background-color-end}', GradientType=0);
     text-shadow: 0 -1px 0 rgba(0, 0, 0, .25);
   }