Currently it is possible to have multiple UI Modules.
To set them up we need to do 2 things in a project.
reroute the custom variables and exclusions to your CLIENT_core
and in project settings change the load order in the theme tab.
I would love to see a way to automate this.
by adding a master,json file in your UI module, MendixPro would understand that this is the module that contains the variables & Exclusion variables.
Mendix can then, reroute the files in theme to the right folder in themesource.
the second would be to add a loadorder.json file where the master.json file lives, so that modules can be loaded in the correct order (and if they dont exist, this wont cause a bug), which is why it would need to be a json file and not sass.
Lets have a discussion on how to create one click deployment for styling modules.
(and dont get me started on styling dependancies)