Use variable instead of color code

hypha/static_src/src/sass/normalize.scss

@@ -4,6 +4,8 @@
 /* Document
    ========================================================================== */
 
+@import 'apply/abstracts/variables';
+
 /**
  * 1. Correct the line height in all browsers.
  * 2. Prevent adjustments of font size after orientation changes in iOS.
@@ -231,7 +233,7 @@ fieldset {
   padding: 0.35em 0.75em 0.625em;
   border-width: 1px;
   border-style: solid;
-  border-color: #cfcfcf;
+  border-color: $color--mid-grey;
 }
 
 /**