Add fade animation

core/static/core/accordion.scss

@@ -37,3 +37,32 @@ details.accordion>.accordion-content {
   border-bottom-right-radius: 3px;
   border-bottom-left-radius: 3px;
 }
+
+@keyframes open {
+  0% {
+    opacity: 0
+  }
+
+  100% {
+    opacity: 1
+  }
+}
+
+/* closing animation */
+@keyframes close {
+  0% {
+    opacity: 1
+  }
+
+  100% {
+    opacity: 0
+  }
+}
+
+details[open].accordion summary~* {
+  animation: open 700ms;
+}
+
+details::not([open]).accordion summary~* {
+  animation: open 700ms;
+}