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A better and more general page would be: https://live.gnome.org/Design
and for apps
For a music / media player:
Even more (recent) mockups can be found on https://github.com/gnome-design-team/gnome-mockups
Note that new GNome design seems to break radically with older implementations (Banshee, Rhythmbox, ...), and other OSes.
Thanks. Reading the HIG would be step one, of course. I was really looking for some tips that may not be so easily found in written guidelines. The kind of tips you only get after really using a system for a prolonged period of time.