Customising the reply buttons

I’ve made a few minor changes to CSS so the theme works more to my liking. Most of it is trivial, but it seems unpicking the CSS around the reply boxes is more complex as there are a number of moving parts.

What I’d like to do is replay the full colour blocks with either black text on a white background or, if that’s too hard, use the theme primary colour for the text.

How would you go about this?

The page I need help with: [log in to see the link]