So I get that it may not be easy to do this, or might not seem useful, but .... right now I'm actually contemplating never rewriting any default CSS and literally creating all my own classes to override styles because there is so much work still involved in upgrading from BS3 to 4 in our custom css files. Yes I can kind of do search and replace all specific things, but I can't even find a find/replace setup in the CSS window, it seems to only apply to the HTML window.
So, it seems that only the default CSS file is updated when we upgrade from 3 to 4. Since it's making a copy, what have we got to lose if it updates classes and ID's within the HTML and our Custom CSS file, or even give us the option to put a check next to the files we want to include in the upgrade? Since we have the original still intact, we can always go back and do it manually if it isn't right, but I'm guessing the parts that might not be right will be fewer than the parts we have to manually do now.
My example here is the pain in my arse with changing one simple little Panel to Card, so many little places I have to change it, yet if the upgrade searched specific files for the new changes as well, it may save us a lot of work.
I'm willing to help test this theory if you need a tester for it, but I'd really love it if it went a little further than it's going for the upgrades.
Once again thanks for any consideration you can give to this.