Use CSS vars for .s-modal__full

@aaronshekey yell at me if we shouldn't do this

Author: dancormier

lib/css/components/_stacks-modals.less

@@ -128,6 +128,6 @@
 //  $   SIZES
 //  ----------------------------------------------------------------------------
 .s-modal__full {
-    max-width: calc(~"100% - @{su48}");
-    max-height: calc(~"100% - @{su48}");
+    max-width: calc(100% - var(--su48));
+    max-height: calc(100% - var(--su48));
 }