We love it when new utility applications are created to work with existing applications. An example of this is newly-released SVDesignMiner from To•The•Point Software. It browses the Sandvox designs on your system and makes it easy to duplicate an existing design and start modifying it to your tastes.
It doesn't hide the CSS files from you to make minor adjustments; that is an entirely different ballgame. But if what you want to do is tweak a design, especially the graphic elements that make up an existing design, this could be a useful tool!
By the way, a couple of other companies will be releasing their own custom Sandvox designs. If you are looking for some designs that go beyond the forty or so available from Karelia, you may want to give those a try. We'll mention them here when they go online.