Premise: How do I rebrand the login page without updating the backend files?
Solution: Should you wish to assign custom department and product logos to your main login page (URL accessible to all departments) without having to change the back end files you can update one department to have the logos and assign the main login page URL as the custom domain.
-Log in as superuser
-Navigate to ADMINISTRATION > DEPARTMENTS
-Left-click the 'Themes' link under SuperUsers
-Provide the custom logos
-Assign the main login page URL in the Custom domain field
-Click 'Update Department'
-Clear your browser cache or navigate to your main login page URL and refresh the page
-Any user now accessing the main login page will now be viewing the new custom logos (make sure to include http:// in your browser's URL bar)