Yes, login to whm, scroll down to the "Modify an Account" option, select the user you want to have the mail-only theme, click ok, then on the theme dropdown, select "xmail", click save, and you're done.
I've gone to Account Functions->Modify Accounts, selected the account and could then have applied the xmail theme. The only problem is that I need one user to have be xmail and another user to have access to the entire functionality of WHM. I looked for a "add user to account" option but I could find none.
So what you're looking for is to have one domain with two users that can access the cpanel for that domain: one user can access full features with the x theme and one user can access just email features with the xmail theme?
Well, I know you can't do it in httpd.conf... adding any type of additional user in here for an account will cause apache to fail. Maybe someone else might know if this can be done?
I tried this myself and found that when you copy the file in the var/cpanel/users folder, then the domain is listed twice in whm's user management... this is why the email addresses aren't showing up... cpanel thinks there's two accounts with two different domains, even though the domain is the same... I'm working on this too...