The login for all the trac's is the exact same login as here on the Support forums.
Username is indeed case sensitive, but your username (richarduk) is all lowercase anyway. Password is the same as well. Changing your password here on the forums will change it there on trac too. There's no separate registration, we only have one unified system for usernames and passwords.
Your theme ticket is here:
And yes, you do need to login on trac to leave comments and such.
If you're having trouble logging in, change your password here on the support forums. The change *should* take effect immediately, I think, but don't hold me to that. It may take up to an hour for the change to be reflected in trac.