I didn't feel like there was much inovation and graphics-wise I always feel a bit uninspired by simple-geometry games - but the gameplay mechanics were really fun and interesting, and the feel of the gameplay is simply perfect! I love the feedback and the difficulty-curve, which is perfectly balanced and I never got surprised when hitting enemies or loosing life. That was so well done! Perfectly polished and a really great entry!
I couldn't really find the theme in more than the actual name of the game though. ^^
The upgrade system was also extremely fun, and made the game replayable over and over. I love how you're introducing the perfect amount of complexibility without the player having to stop and remember a lot of specifics of all the enemies and such. Great work! :)