Custom styling in Mendix directly in Studio Pro. From version 9.9.0 on, an integrated style editor is available in Studio Pro. As soon as you convert a module in your project to a so called UI module, the Styling option will become available (see the animation at this post).
Beginning with version 9 (in conjunction with the new Atlas structure), it was already the case that Studio Pro automatically compiled your stylesheets. So Koala or Gulp was already no longer needed. This change makes it easier again and you no longer need to switch to another tool to adjust the styling.
A great addition as far as I’m concerned!