First, after reading your description, there's nothing wrong with the graphics. They serve their purpose. You should see mine! Similar thing. Second, completing a game is hard. Completing a game in seven days is harder. Completing a game in seven days that's functional is a nigh-impossible feat. A lot of submissions don't work at all, or don't have anything uploaded. You should be proud of what you accomplished.
You have the fundamentals of a game down. Before working on aesthetics, try to tune it a little more. I think better feedback that hitting the red obstacle was the "win" condition would be beneficial. Green vs. red is a good start, but I thought I just lost even though I intentionally hit it. Also, the game is far too easy -- although there are worse problems to have, like it being frustrating or unfair. Play with the settings a little more and see if you can't make something a bit more challenging.
I also would have used a longer music loop, but that isn't something you made. No shame in that, as the people that are able to create 100% of the assets in seven days are few in number, and are usually very experienced game developers.
Overall, this is a great start, and far greater of one than when I started. Keep it up! Good job.