Multiple Sites / Changes

I have been really impressed with the Epik theme and all the useful info in this forum. I appreciate all the tips and conversation. That said: I’m new to web design etc., and wonder if there is a “smart” way to save css and function.php changes so that they are usable on additional sites, and also how those changes are affected when there is a theme update. Should I be creating sub-child Epik files? Not sure if I’ve phrased this well, but hopefully someone will get my questions. Thanks!