I think you’ll need to manage the Devise views yourself. Try the following in a console:
rails generate devise:views
This will generate all the views Devise uses (including mailer templates), which you can now customize.
The mailers you’re looking for should then be in ‘app/views/devise/mailer’
If you want to generate scoped views, or only a subset of them that is also possible. Per the documentation at https://github.com/plataformatec/devise#configuring-views:
You can also use the generator to generate scoped views:
rails generate devise:views users
If you would like to generate only a few sets of views, like the ones for the registerable and confirmable module, you can pass a list of modules to the generator with the -v flag.
rails generate devise:views -v registrations confirmations