Anybody out there know CSS?

I’m trying, again, to create a set of child theme .css files so that the NEXT time they update the theme I don’t have to drive myself nuts fixing things again.  I want to over ride the font-family code, the problem is that font-family is designated in at least 10 different spots in 3 different files.  Now I know I have to create seperate child.css files for each of the individual files, but I don’t want to have to do 10 sets of code for each one either.  In theory:

    font-family:Book Antiqua!important;

at the top of the child file will override the rest of the font-family designations.  Is that correct?