Christopher,
Glad that helped. What portion of the modal are you talking about? The actual popup, or the Login | Register text links, or something else?
The modal is a custom design, and the CSS is a little scattered, but mostly in our template.css file. You're free to copy it however you want, but we don't have template or setting to switch to our the layout we use here. Our popup is also not currently responsive. The SCLogin modal popup normally is. Our CSS on this site breaks the responsive-ness, for now, but we're going to be fixing that over the next week or so as well.
Hope that helps explain a little, but if you need anything else, let me know.
Thanks,
Alex