replace deprecated slash division with math division in Sass

Change-Id: I664c367e06eb166f74197ac50ef90dc24a7b5ef9
diff --git a/dev/scss/base/flextable.scss b/dev/scss/base/flextable.scss
index dc5ffd9..f10cec3 100644
--- a/dev/scss/base/flextable.scss
+++ b/dev/scss/base/flextable.scss
@@ -1,3 +1,4 @@
+@use 'sass:math';
 @import "lengths";
 @import "mixins";
 
@@ -20,7 +21,7 @@
     flex-direction: row;
     text-align: center;
     width: 100%;
-    border-width: $border-size / 2;
+    border-width: math.div($border-size,2);
     border-style: solid;
     align-items: stretch;
     > * {