In that case, you would want to go with a custom login theme rather than a custom webmail theme. I discussed how to create a custom login theme during my Branding presentation at the 2008 cPanel...

What, specifically, did you want to customize:

- The actual login form where users enter their email address and password
- The screen where users select Horde, SqirrelMail or Roundcube

And/or...

In the WHM interface, in the Cluster/Remote Access section click on Setup Remote Access Key. Here you can retrieve your current access key or generate a new key.