BuddyPress.org

This is not a BuddyPress topic and should be targeted at the WPMU forums instead. But I’ll give you a little help nonetheless.

You should be using an IDE (integrated development environment) that allows you to search all files for the piece of code you want to find–among many other very useful features. I’m on a Mac and I prefer Coda to TextMate–although I used to use the latter.

The CSS is actually near the top of the wp-signup.php file. Search for function wpmu_signup_stylesheet().

I’m also setting this to “not a support question” as it has to do with styling in a WPMU file and nothing to do with BuddyPress.