It's open source. You don't even have to talk to the developers if you'd rather not. Fork it and make it beautiful, and release it. The world will thank you. If there's enough demand, the devs will merge it into the original project.
If I was a designer and not a programming type, this would be extraordinarily difficult, especially when it comes to refactoring someone else's code and interface to fit my vision.
Indeed design is not about sticking a pretty UI on top of the existing software.
You have to apply UX to the development process, such as how a user interacts with features, or the steps involved in each action, or the way the app communicates to the user, or delivering feedback after an interaction is completed. Design influences the software requirements.
It's necessary for designers to collaborate with developers if you want a well designed app... not just a pretty one.
Or ruin it...