Here's an interesting trick I discovered while using GNOME Shell a while ago. This video was made using GNOME Shell 2.91.92, so I'm sure it still works at the time of this writing.
Basically, I discovered that you can have limited window titlebar theming in GNOME Shell while you are using the Clearlooks theme. By changing your window background color, you can also change the colors of the Adwaita titlebars. It's very quick and simple to do, and can give your Shell a bit of personalization.
UPDATE: This no longer works in GNOME 3 due to changes in preferences.
