Hm, and so this is completely out of date: Customize the appearance of the user portal | Yunohost Documentation
We used to have our own theme, but it got broken when we upgraded to version 12. It seems like the code changed completely: [Theme] Dark Horizon theme - #8 by ljf
There are no official hooks for us, so hacking the code means modifying yunohosts file and when there is an upgrade, it will be overwritten? Is this by design or is it an omission that would be eventually added back? A login page with no way to tell users where to reset password is really bad UI in my opinion :-(.