Made the split separator darker

This is a fix for #9146.

@RaymondLim I've made the separator a little louder for light and dark UI. Hopefully these will do otherwise it'll start breaking the visual hierarchy.

![paneheaders](https://cloud.githubusercontent.com/assets/1542488/4339394/51180ade-4023-11e4-8d93-1b6d915eca62.png)