In open source projects, why do software patches have to be accepted by a developer? Couldn't contributors just release their own patches and allow people to add the patch if they choose to. I'm wondering why there are so many Ubuntu programs that could use such obvious work but aren't updated.

For instance, lots of people want Rhythmbox to be more attractive. Why can't the people who design themes on DeviantArt turn those into code and let users download those themes themselves, even if the developers at Rhythmbox won't accept different themes?