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.
Working on a Mac in a VM, MX-9 does not seem to notice changes made in the styling. The watcher is not triggered... Every now and then it does, but that is almost 50-60 sec after the change is made (!?) All in all : the SASS compilation is not working for me. Is there some fix for this?