Fix scss warnings

ui/src/design/_typography.scss

@@ -147,7 +147,7 @@
   $max-screen,
   $max-value
 ) {
-  $a: ($max-value - $min-value) / ($max-screen - $min-screen);
+  $a: calc(($max-value - $min-value) / ($max-screen - $min-screen));
   $b: $min-value - $a * $min-screen;
 
   $sign: '+';