Thank you Andrew for the information
I seek to be able to 'deliver' a different theme according to the "role" of the logged user. my need is for 'developpement' purpose where I would have a wait page theme for all user but one 'on work' theme for the admin to be able to get use to it 'live'

I understand that this must be done at a plugin stage, before theme is loaded which is obvious but I have no idea where to start

any tips?

thank you in advance