grid-background vertical rhythm should be based off of $blueprint-font-size, rather than a static value.

Closes GH-429.

frameworks/blueprint/stylesheets/blueprint/_debug.scss

@@ -14,7 +14,7 @@
       $total    : $blueprint-grid-columns,
       $column   : $blueprint-grid-width,
       $gutter   : $blueprint-grid-margin,
-      $baseline : 20px
+      $baseline : $blueprint-font-size * 1.5
     );
   }
 }