Use default cpp fmt build script.

vendor/Fmt/doc/bootstrap/mixins/opacity.less

@@ -0,0 +1,8 @@
+// Opacity
+
+.opacity(@opacity) {
+  opacity: @opacity;
+  // IE8 filter
+  @opacity-ie: (@opacity * 100);
+  filter: ~"alpha(opacity=@{opacity-ie})";
+}