Add box shadow to Modal so all existing overlay components are elevated above surface

Author: adamkudrna

src/lib/components/Modal/Modal.jsx

@@ -290,7 +290,7 @@ Modal.propTypes = {
    */
   onClose: PropTypes.func,
   /**
-   * If set, the modal is rendered in the React Portal with that ID.
+   * If set, modal is rendered in the React Portal with that ID.
    */
   portalId: PropTypes.string,
   /**

src/lib/theme.scss

@@ -877,7 +877,7 @@
     // =====
 
     --rui-Modal__background: var(--rui-color-white);
-    --rui-Modal__box-shadow: none;
+    --rui-Modal__box-shadow: var(--rui-elevation-2);
     --rui-Modal__outer-spacing--xs: var(--rui-spacing-2);
     --rui-Modal__outer-spacing--sm: var(--rui-spacing-6);
     --rui-Modal__footer__background: var(--rui-color-gray-100);