For those building gnome3 frequently from git or paying attention to gtk-devel, this might be old news, but still worth mentioning… The gtk-style-context branch has landed in GTK+ master, which will quickly unveil a powerful theming system, powered by CSS files.
Although some apps (mostly these implementing widgets themselves) might undergo some work to adapt to this, there are API docs and migration documents (mostly courtesy of Matthias, who’s done an excellent work) that should ease the task.
Besides the GTK+ work, there is also both a gnome3 theme and engine using the new stuff. Things are still ongoing here, as many things can be leveraged into CSS, which would leave the engine very little things to do. Andreas started looking at some point into the branch, so he might have some things ready in that field :).