Previewers: Ensure popup is visible across themes (#3080)

Author: hoonweiting

plugins/previewers/prism-previewers.css

@@ -13,6 +13,7 @@
 	width: 32px;
 	height: 32px;
 	margin-left: -16px;
+	z-index: 10;
 
 	opacity: 0;
 	-webkit-transition: opacity .25s;