What happens if you try:
<iframe style='border: 0px; width: 100%; height: 100%;' scrolling='no' src='https://mail.mailserver.com/?skin=mycustomskin'></iframe>
Where 'mycustomskin' is the name of the custom theme you have created. That should get you the pre-login screen with the desired theme.
Note the Zimbra licensing requirements re branding; they are different for the Network and Open Source editions.
Mark
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___________________________________ L. Mark Stone, CIO  "Uptime. All the time."
477 Congress Street | Portland, ME 04101-3431 | (207) 772-5678
proactive maintenance and monitoring | technology consulting
Zimbra groupware | EMR implementations | private cloud hosting
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