Child Theme Access For Each Blog User

Good Day Everyone,

I am wondering what I need to setup in order to allow each user to have his own editable css for each child theme?

Does anyone know is this is an existing functionality in WPMU, or if there is an exisiting method to do this?

Also, I would like to make sure each blog has it's own folder for uploads and css.

Can anyone send me in the right direction on these ideas?