To customize how Devise sign-up, edit profile, and sign-in forms look, you run the command
rails generate devise:views
which creates a folder in app/views called devise and extracts all the view templates that Devise uses out of the gem in to it. That way you can customize them to your heart's content.
However, I have prepared simple, Bootstrapped templates for each; feel free to steal them. Just replace the files you generated with the ones in this repo, or copy the code in to each one.