Set fixed theme

package.json

@@ -96,7 +96,7 @@
         "c9.ide.openfiles": "#7fa4a97fed",
         "c9.ide.preview": "#23b5e2149d",
         "c9.ide.preview.browser": "#cec211a76a",
-        "c9.ide.preview.markdown": "#fa98ff2743",
+        "c9.ide.preview.markdown": "#bc846e1562",
         "c9.ide.pubsub": "#a85fb27eca",
         "c9.ide.readonly": "#e67bb593bd",
         "c9.ide.recentfiles": "#7c099abf40",