My design skills are awful, but using Inkscape [2] I was able to come up with something half-decent (according to my friends, assuming they're not just humoring me). I found Derek Banas' youtube tutorials [3] very handy for learning how to use Inkscape.
Incidentally, one very useful tip I found (can't remember where, maybe stackoverflow) is that you must avoid creating any new objects during the render() method (ie during the game), because you'll get unpredictable garbage collection, making the game pause at random intervals. A handy way to check you haven't missed anything is to put a breakpoint in the Object class constructor in android.jar. If it gets hit during play when running in debug mode on a real device, then you're creating an object when you shouldn't be.
Feedback / suggestions on this game very welcome. Thanks!
[1] http://www.packtpub.com/learning-libgdx-game-development/boo... [2] http://www.inkscape.org [3] https://www.youtube.com/watch?v=zUIOEXssTSE