As someone from the desktop customization scene in the Linux world, I can tell you that it is not entirely dead and still has some actual activity... if you aren't using GNOME. With GNOME 2.x, you could do things via GTK themes and Metacity themes before compiz came around. But with GNOME 3... they threw away as much customization as possible for the same bizzare reasons they had for creating an alienating UI. Can't really do window theming if you're overloading the window title bar with extra UI elements. Though the default GTK theme was initially a banger; it was made by the same guy that created the Cairo theme engine and it looked really good. But now he got rid of that and replaced it with... flat garbage. It's so sad.
If you use KDE, all the theming is still supported and there are still people involved in it. If you use any of the other GTK-based DEs (like XFCE) you can still do window decorations but GTK itself is still locked down.
