Fix dismissible-box styling issue (#2564)

Fix dismissible-box styling issue

Changes in #2487 renamed the class from alert-dismissible to
alert-dismissible-box, causing Bootstrap CSS styles on
.alert-dismissible .btn-close to lose effect.

This commit restores the original styling by adding back
the necessary CSS styles to .alert-dismissible-box .btn-close.

packages/vue-components/src/Box.vue

@@ -355,6 +355,10 @@ export default {
     }
 
     .alert-dismissible-box .btn-close {
+        position: absolute;
+        top: 0;
+        right: 0;
+        z-index: 2;
         padding: 1rem;
     }