Fix variables defined in variable.less cannot be overwritten (#6819)

Signed-off-by: Jack Cherng <jfcherng@gmail.com>

skins/elastic/styles/variables.less

@@ -11,7 +11,6 @@
 
 @import (reference) "fontawesome";
 @import (reference) "colors";
-@import (reference) "_variables";
 
 @screen-width-large:    1200px;
 @screen-width-medium:   1024px;
@@ -55,3 +54,5 @@
 // Additional icons
 @icon-resize-corner:    data-uri("image/svg+xml;charset=utf-8", "../images/corner-handle.svg"); // size: 16x16
 @icon-file-drop:        data-uri("image/svg+xml;charset=utf-8", "../images/download.svg");
+
+@import (reference) "_variables";