no CSS loaded

Are you able to send me a link to your site?

Maybe it has errors merging the CSS.

https://osteopathe-biomecanicien.com/ but I had to rollback the install in order to have a running website (I’m using the Doctors theme)

Were you using the Minify plugin at the same time? If so I believe that is the cause of the problem. When I tried the Doctor theme with just MMR it seems fine.