Every control in the new version of simple styles takes a depedency on a file called Shared.xaml. This file contains a set of resources (mostly brushes.... err, actually, all brushes) that define all of colors that I use. I was pretty strict about everything in that file and not hardcoding anything. The result of this is that it's pretty easy to make a new color scheme for simple styles.
To test this out, I took a stab at creating "glassy" version of simple styles. Here's the updated shared.xaml. It mostly worked. Here you can see some of the original simple styles (on top) and the glassy version (below).
There are a couple of places that I ran into some problems, though. I made the "WindowBackground" color semi-transparent because I wanted TextBox and ListBox to get the glassy feel. The problem is that I use this same color as the background for the popup in Menu and ComboBox. A semi-transparent Menu is pretty tough to use. I also discovered that I had hardcoded some colors in ScrollBar. This will need to fixed in a future version.
To actually use this, you'll probably have to make a handful of tweaks in the SimpleStyles themselves. I'll leave that up to you. Overall, though, you could be up and running with this in 10-15 min. The whole thing took me about 45 minutes... not bad for creating a whole new look for Simple Styles.