I didn't copy core.css into custom.css- I only put the changes in custom.css and I think that is the way it is meant to work? You don't need to comment out core.css because it takes the main code from there and only the customised styles from custom.css
It makes it much easier to work with because it is a much smaller amount of code.
It makes it much easier to work with because it is a much smaller amount of code.