Near the bottom of the Atahualpa Options you'll see "Various Content Items". The last option there is "Add html/CSS Inserts"
This is where you can add the CSS you want for your h1, etc.
That's what I love about this. It's just adding the CSS here that I want. No modifying.
I've even modified plugin CSS here, so when a plugin gets updated, my changes still hold true.
Velma