Until adobe creates the spark analog of mx|datagrid, I think your only option is to skin old school style.
Googling 'flex skinning scrollbar' netted a bunch of good results.
I tried running these apps and they worked fine on FB beta (well, you might have to update one of the .css files). Apparently you still have a measure of control over the appearance of the scrollbars, though the work is considerably more involved than skinning spark components.
Any word on Adobe creating a Spark version of the DataGrid?
The Halo DataGrid (and the Tree and probably some other components,List comes to mind) is not being properly styled with Beta 2 of Flash Builder / SDK 4. Some elements are styled correctly and other not at all. It's Very Annoying to say the least.
Is there a roadmap posted for the creation of additional Spark components (preferably with timeframes) posted somewhere?