for now, we only support basic selectors, when mixing in, eventually, we can upgrade the Selector/match method to return the number of matched elements

test/less/mixins.less

@@ -13,8 +13,10 @@
     }
   }
 }
-#theme > .mixin {
-  background-color: grey;
+#theme {
+  > .mixin {
+    background-color: grey;
+  }
 }
 #container {
   color: black;