Fixed fallback background color for old IE buttons

Author: alienresident

templates/shared/sass/partials/_mixins.scss

@@ -5,7 +5,7 @@
 @mixin simple-background-gradient($color, $background-gradient-pos, $background-gradient-1, $background-gradient-2) {
   color: $color;
   // provide background color for older IEs
-  background-color: background-color-mix($background-gradient-1, $background-gradient-2);
+  background: background-color-mix($background-gradient-1, $background-gradient-2);
   @include background-image(linear-gradient($background-gradient-pos, $background-gradient-1, $background-gradient-2));
 }