At the present time, it is not so easy to customize the web login page of the captive portal.
In any case you could change the /root/kerbynet.cgi/template/cp_showauth file which contains the web login page and then, to make the changes permanent you should copy it in the /Database directory and put the command
cp /Database/cp_showauth /root/kerbynet.cgi/template/
in the file /etc/rc.local that is executed at the startup.
Regards
Fulvio