How to reset GP Completely and start fresh?

Hello

So, I had GP in a site and switched to a different theme for quite some time (experimenting). Now I am moving back to GP. As soon as I installed GP, everything came back to exactly how it was when I originally had GP previously. All the custom CSS, settings, everything - they are all there.

The thing is, I want to start fresh. With GP 3 bringing so many improvements (and also to get a new look), I want to start building from the scratch. Is there any way to completely reset GP? Any and all advice on this would be great.