Welcome to the forum.

It's not that easy to customize login/registration pages. The files you may want to check are:
/rsc/css/login.css
/skins_adm/login/_html_header.inc.php
/skins_adm/login/_html_footer.inc.php

I found the files that you said

where can I find " $copyright_text" ?

should be in /conf/_application.php

Thank you for that, it was a great help

no problem