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No amount of theme engine is going to make Clementine look anything like a regular GNOME/Ubuntu app. Sure, it's powerful, but it's also a cluttered mess.
How is clementine cluttered? Serious question, not looking for flames. It's a simple two-pane layout with the library on one side and the playlist on the other. (Or have I configured it that way? Can't recall, it's been a couple months since I installed it; I just play music with it.)