Light gray bg for white code scheme if used in comments and wiki

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/assets/stylesheets/highlight/white.scss

@@ -186,3 +186,11 @@
     }
   }
 }
+
+.readme-holder .wiki, .note-body, .wiki-holder {
+  .white {
+    .highlight, pre, .hljs {
+      background: #F9F9F9;
+    }
+  }
+}