fix: Switch to a variable from pixels for font-size of col sorting index text

scss/grid/_theme.scss

@@ -94,7 +94,7 @@
         }
 
         .k-sort-order {
-            font-size: 12px;
+            font-size: $icon-size * .75;
             margin-left: -4px;
             line-height: 1;
             vertical-align: text-top;