HI Łukasz Walczak,
There is not a lot of information about the changes that this Core Delta module provides. But I was able to find that the purpose of the module is to make additional changes on top of the current styles of the Atlas Core module for your Mendix app.
It seems like they are refactoring some styles that were not needed:
I also see that they are also adding alternative attributes for css styling, probably for more compatiblity for different browsers:
I hope that helps with your curiosity :) , regards!