Preferred way is first file an issue to issue tracker to discuss and review the the idea that you want to do.

Byt yes, we will allow pull requests also to UI side of code, but those pull requests needs to be approved by our designers. Thats why its better first file issue to issue tracker to discuss the changes. Ie. to avoid possible disapointments that you make a huge pull request just to be rejected if it conflicts the design documentation (which will also be available soonish https://github.com/sailfishos/sailfish-browser/issues/2).