_header.scss: Fix Dart Sass warning (#3915)

Author: XhmikosR

layouts/css/page-modules/_header.scss

@@ -102,7 +102,7 @@ header {
         pointer-events: none;
         border-top-color: $node-gray;
         border-width: $border-width;
-        margin-left: -$border-width / 2;
+        margin-left: -$border-width * .5;
       }
 
       &.active:first-child::after {