Modification of email template
Go to admin/config/people/accounts, Under Emails click Account Activation and then change as per your requirement. Use html and css for customization. See the below image 



(or)

Use the second option below:

All the emails are being sent from the system and the content is editable as we want. To stylize the emails, use html mail module http://drupal.org/project/htmlmail

This module allows you to theme emails leaving the system as you would theme a normal Drupal site, and replaces the outgoing emails with those HTML versions per your settings.

hope it helps you!!!