Use import (reference) whenever the less file supposedly emits no CSS

customize.dist/src/less2/include/leftside-menu.less

@@ -1,6 +1,6 @@
-@import (once) "./unselectable.less";
-@import (once) "./variables.less";
-@import (once) "./colortheme-all.less";
+@import (reference) "./unselectable.less";
+@import (reference) "./variables.less";
+@import (reference) "./colortheme-all.less";
 
 .leftside-menu_main() {
 }