fix code block style in markdown preview

package.json

@@ -96,7 +96,7 @@
         "c9.ide.openfiles": "#7fa4a97fed",
         "c9.ide.preview": "#72a4521169",
         "c9.ide.preview.browser": "#cec211a76a",
-        "c9.ide.preview.markdown": "#da9d9c65cd",
+        "c9.ide.preview.markdown": "#5081170bfb",
         "c9.ide.pubsub": "#a85fb27eca",
         "c9.ide.readonly": "#e67bb593bd",
         "c9.ide.recentfiles": "#7c099abf40",