How would you compare Superset with PowerBI for analytics and CSS integration? Trying to develop features and advanced analytics capabilities into an app?

You can style dashboards with CSS as much as you'd like, though there are some limitations (canvas/webGL elements). I wrote a whole blog post on it: https://preset.io/blog/customizing-superset-dashboards-with-...

If you want to style the whole application, you can fork the repo and go bananas. If you're looking for theming, there's more to be done yet on that front, and I wrote an article on that too: https://preset.io/blog/theming-superset-progress-update/