Which is better really depends on the function and content of the app. I prefer dark Ui for media heavy or editing software (like Affinity Designer) where the content is the star and the associated ui or tools are really only supplementary to that media. I reserve light interfaces for text or navigation heavy experiences where the focus is on scanning or consuming text.

There are definitely arguments to be made based on average user session time as well. Longer sessions definitely skew in favour of darker interfaces (or at least not straight white) as they're easier on the eyes long-term.

I've been using Zeplin with my team for almost 2 years and love it. We've tried everything out there and keep coming back to Zeplin. It's just the best.

It wasn't so long ago we were producing implementation guides for mocks in InDesign as 80+ page booklets. Despite its bugs, Zeplin is 1000X better than that workflow. It may not be perfect, but us old timers appreciate the time savings.