replace deprecated slash division with math division in Sass

Change-Id: I664c367e06eb166f74197ac50ef90dc24a7b5ef9
diff --git a/dev/scss/header/header.scss b/dev/scss/header/header.scss
index 76c583e..c0c7469 100644
--- a/dev/scss/header/header.scss
+++ b/dev/scss/header/header.scss
@@ -1,4 +1,5 @@
 @charset "utf-8";
+@use 'sass:math';
 @import "../util";
 @import "hint";          // Hint specific menu list
 @import "containermenu"; // Container menu specific
@@ -86,7 +87,7 @@
       top:              0;
       right:            0;
       margin-right:     0;
-      width:            ($standard-margin / 2);
+      width:            math.div($standard-margin,2);
       background-color: $dark-green;
       text-align:       center;
       height:           100%;