The phplist is a great mail list software.
But I found that it is difficult to design the subscribe page.
Because all pages code are in "index.php" file.
If you want to replace the subscribe page, the unsubscribe page needs to be updated too. And some related messages pages need to be updated.
So you need to change "index.php" and "admin/subscribelib2.php" file.
From my understanding, I think the UI and implementation is not separated.
So I suggest to separate the UI and implementation code.
Use some separate pages for subscribe and unsubscribe page and other message pages. Then it will be very easy for the user to design them without code change.