Is it possible to use Prismic as a page builder for a multi tenant app? While still still allowing some choice like logos, colors, fonts, etc.
Welcome to the community 
If you'd like to use Prismic for different websites and projects, you're absolutely able to. Your account can have completely separate repositories where different projects are housed, linked to different code bases and with different users in them. Any user can have access to several repositories (they will all appear on your dashboard), and you have complete freedom on whether any of these repositories are linked/related to each other in your code or elsewhere.
I hope that answered your question