Actually, with the new login theme, you also have the option to create a new login theme, it just isn't available in the GUI for some of the steps that I could find.
The login themes are located at /usr/local/cpanel/base/unprotected folder (these are cpanel, cpanel-legacy and mobile). If you copy the cpanel one, you can then make changes to it and switch to using that theme.
First copy it and call it cpanel-new for example:
Go to WHM > Tweak Settings, and now select cpanel-new in "Default Login Theme"
cp -R cpanel cpanel-new
Make your changes to the files, then upload the changes and they should show up for the theme. You might have to rebuild the sprites, not sure on that as I didn't make any file changes at this point.
I agree this is a good feature request for branding it, but hopefully the above is a better way to change the login theme only without impacting the cPanel theme.