[Plugin: Secure Invites] Customize Invite Code Required Page?

I’d like to edit the page that visitors are taken to to input an invitation code, so that it looks like some custom login pages. To be specific, I’d like https://caskers.com/register to look more like https://caskers.com/welcome.php. It looks like all the CSS in on invitation required page is coming from install.css or install.dev.css, but when I edit those files, the invitation required page does not change.

https://www.ads-software.com/extend/plugins/wordpress-mu-secure-invites/