Make chevron rotate

layouts/shortcodes/accordion.html

@@ -27,6 +27,15 @@
   }
 </style>
 {{ end }}
+<style>
+  #{{ $id }} details[data-accordion-item] > summary .accordion-chevron {
+    transform: rotate(-90deg);
+    transition: transform 200ms ease-in-out;
+  }
+  #{{ $id }} details[data-accordion-item][open] > summary .accordion-chevron {
+    transform: rotate(0deg);
+  }
+</style>
 {{ if eq $mode "collapse" }}
 <script>
   (() => {