re-add modifier to header to be able to change header icon colours

opentech/static_src/src/javascript/components/search.js

@@ -20,6 +20,9 @@ class Search {
         // swap the icons
         this.node[0].querySelector('.header__icon--open-search').classList.toggle('is-hidden');
         this.node[0].querySelector('.header__icon--close-search').classList.toggle('is-unhidden');
+
+        // add modifier to header to be able to change header icon colours
+        document.querySelector('.header').classList.toggle('search-open');
     }
 }