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.
Not sure what percentage of brightness you're referring to as normal. The following screenshot istaken with 70-75% brightness and I still think border-bottom of the pane headers is hard to see.
![paneheaders2](https://cloud.githubusercontent.com/assets/1542488/4339971/60cb6754-4029-11e4-85c1-06e8773d42bc.png)