These are important new features, and I'm really glad you have managed to implement them. The start on generic classes for box-shadows puts more power and flexibility in the hands of theme and palette creators, and makes it easier to create palette compatible themes. It will be good to extend this concept for background/foreground colour combinations.

Getting the preview images working again is important. But the way you've implemented is great. Again, I'm excited to play with the possibilities this new method allows in terms of theming.

And the other styling, theme and palette additions are great. I'm especially impressed with the social menu colour filters. An elegant and powerful solution!