If the new UI resources module is just an add-on to Atlas-Core this should work.
However, if the new module is a complete replacement for Atlas-Core you will need to remove the Atlas-Core module.
Marking a Module as a UI resource does not affect IF styling from this module is loaded. This setting enables the editor features in Studio Pro and allows you to define the load order of such modules.
So in your case, probably all the stylign from the themesource/atlas_core is still loaded.