Gmail feed with a Google Apps email

In Gmail click on Mail Setting and then Themes. You can choose from some pre-created one or create your own.

EDIT:

Try this link on which you have a button (Explore themes) to access the Themes settings under gmail (make sure you're logged in to gmail): http://www.google.com/support/bin/static.py?page=guide.cs&guide=28576&answer=180095

Your domain may have been upgraded. If so, you should still be able to login from http://mail.google.com. Just enter your full email address (not just your username) when you try to sign in.

As an admin, you should also be able to login to your cpanel at http://www.google.com/a/example.com and acces your mail from there; while visiting your cpanel, you can confirm whether your domain has upgraded and force the migration if necessary.