theme: apply modal-light to playground modals on samples/index and injected changelog fragment; ensure no modal_playground leftovers

Author: matteobruni

samples/index.html

@@ -833,7 +833,7 @@ <h5 class="modal-title" id="changelogLabel">Playground Storage Changelog</h5>
             <!-- content from samples/changelog-modal.html -->
 
             <div
-                class="modal fade modal-playground"
+                class="modal fade modal-light"
                 id="changelogModal"
                 tabindex="-1"
                 role="dialog"