Do I create a custom theme or modify css in a genesis child theme?

I've just got started with Wordpress and it was suggested to use a child theme so I purchased the Magazine Pro theme. This has got the perfect layout for my needs but I find some of the padding is too intense and in particular when I have a leaderboard banner at the top my text for my blog title is wrapping.

I want to reduce my title text and reduce the padding. I know how to do that, I've got plenty of experience in front end dev but I just don't know if I should create a child theme (or grandchild theme) based off Magazine Pro or whether I should modify the css file directly in my Magazine Pro css file? What I'm worried about it overwriting my changes by any updates to the child theme.

What's the best course of action here, I'm really keen to get cracking on some content but my site looks a bit crap with the header text wrapping.