Update recommended scrollbar styling

Hi Trimek,

I've had some recent problems with scrollbars styling in my extension as of late, and for this reason I'd like to update the recommended way of styling scrollbars in your BearTheme.

I've attached a picture to show you what the .CodeMirror-scrollbar-filler and .CodeMirror-gutter-filler that I ask people to style is:
![temp](https://f.cloud.github.com/assets/3637499/1894045/09f54ae0-7acb-11e3-95b7-9624a94a5460.png)

Please let me know if anything is unclear and I'll elaborate.