A couple of thoughts:
First, if you want to quickly try out a CSS change portal-wide, one easy way is to use the built-in stylesheet editor under ADMIN->SITE SETTINGS->STYLESHEET EDITOR. This can be useful for a quick test without going into any CSS files.
Altering USER REGISTRATION
The user registration screen is a dynamically-created screen -- there is no ASCX source containing all the rendered elements. You'll have to go ino VB code or else work backward from viewing PAGE->SOURCE in your web browser to discover page elements and ID values.
This dynamically-generated page presents another issue: What happens when you want to alter CSS only for the User Registration page for elements with no unique IDs? If you add tag-based CSS to your PORTALS.CSS file it will affect your entire site.
USER REGISTRATION = USER ACCOUNT
The User Registration screen is also the User Account screen, so any changes you make will show up for already-registered users whenever they look at their account info. Example: How do you swap the standard DNN "required field" icons for super-special, huge "REQUIRED FIELD" icons during the registration process but NOT swap them later when a user is simply looking at his/her account info?
One Solution: Resource File "Hack"
You might get some useful info from the following post where I detail one way to add content to the user registration page that only shows up during the registration process: http://www.eguanasolutions.com/DNN_Blog/EntryID/3.aspx
You can use the same approach to dynamically change certain CSS attributes via JavaScript. This will enable you to (when it makes sense) alter CSS only on the registration page without rolling the CSS out for the entire portal.
Please come back and update the thread with your thoughts/findings once you've settled on a solution -- I'd like to hear about what you find that works well for you since the solution I linked is really more "hack" than "permanent solution".
Good luck!
-mamlin