Would probably be preferable, since auto makes it behave like no z-index property has been specified (as is the case in the default Bootstrap theme used in lavender and persona) which makes the element inherit the z-index of its parent, whereas z-index: 0 forces the z-index to be, well, 0.
@goik You will need to have the plugin call await user.email.confirmByUid(uid); when the account is created.
But a different question -- do you need to have the account confirmed? By default privileges are set up so that both confirmed and unconfirmed accounts can post new topic/replies, etc., so there is no functional difference except that outgoing emails won't be sent to unconfirmed emails.
If it's merely to "check a box", then maybe it is ok to just not confirm any emails.
Might be the plugin needs updating to auto-confirm email addresses.