What file do I need to edit if I want to customize the box around the message when my "sign up verifications" is on...

The message they see is

"Your account has been created. However, this site requires account activation, an activation key has been sent to the e-mail address you provided. Please check your e-mail for further information."

My problem is my site background is black and the text is white, but the box that around that message is light blue making it hard to see