fix for #1779, use line-height var

less/dropdowns.less

@@ -92,7 +92,7 @@
     padding: 3px 15px;
     clear: both;
     font-weight: normal;
-    line-height: 18px;
+    line-height: @baseLineHeight;
     color: @gray;
     white-space: nowrap;
   }