Add a background color transition for the code sample

src/public/stylesheets/style.css

@@ -854,6 +854,10 @@ html .note-detail-editable-text :not(figure, .include-note):first-child {
     margin-right: 1em;
 }
 
+.code-sample-wrapper .hljs {
+    transition: background-color linear 100ms;
+}
+
 .side-checkbox {
     display: flex;
     align-items: end;