Styling Sign-Up Pages

I'm trying to style the payment pages (and other pages) but can't find the css codes to do that. I've attached a screen shot of one of the pages on my site to give you an idea of the elements that I'd like to change. However, I think all I'm looking for initially is the particular css file for this and, second, a description of the codes that affect the title of the page and the various subtitles.

Thanks for your help.