Styling Field Instructions

Please forgive me if this is a newbie question. I have never bothered trying to style my field instructions until now. I see that I can use HTML and basic inline CSS, which is OK, but I’ve created a lot of different fields and it’d be cool to make my own class for instructions, or mod an existing class. I see that there’s already a class called “field-instruct” which is located in /themes/ee/cp/css/common.min.css. But messing with minified css files is a big bag of hurt and I’m thinking that’s probably not the best way to go - I assume any changes I made would get blown up during updates. Sooooo what would be the best way? Or is there no official way? I don’t want to get all hacky if there’s an easy way to do this.