See the release notes here: https://docs.mendix.com/releasenotes/studio-pro/9.0
Did we mention that the web stylesheets in themesource are defined with Sass? Because they are! And Studio Pro takes care of the mundane process of compiling them when needed, copying used resources, and correctly caching them without the need of external tools like Calypso. The compilation is triggered on file synchronization (via F4), deployment, and module structure changes. When running your app locally, Studio Pro will monitor the theme and themesource directories to recompile automatically when any styling changes are detected.