update backdrop click event

src/css/page-versions.css

@@ -26,6 +26,10 @@
   z-index: 2;
 }
 
+.backdrop.is-active{
+  display: block;
+}
+
 .page-versions h2.title {
   color: var(--nav-heading-font-color);
   font-size: inherit;

src/js/04-page-versions.js

@@ -9,15 +9,17 @@
 
   toggle.addEventListener('click', function (e) {
     selector.classList.toggle('is-active')
-    backdrop.style.display = 'block'
+    backdrop.classList.toggle('is-active')
     e.stopPropagation() // trap event
   })
 
   backdrop.addEventListener('click', function () {
     selector.classList.remove('is-active')
+    backdrop.classList.remove('is-active')
   })
 
   document.documentElement.addEventListener('click', function () {
     selector.classList.remove('is-active')
+    backdrop.classList.remove('is-active')
   })
 })()