Custom CSS on Same Themes on Multi Site

Hope this question makes sense and can get some advice if possible.

I'm Setting up a Site in which to sell my services as a web designer, Currently I have 4 jobs in progress, What I'd like to do is set up new sites on my multisite (Sub Directory) and use back up buddy to move to their own domain once complete.

One moved, I want to keep them on my Domain to use as example of work, My question is where I have 2 or more sites using the same theme on the multisite, How would I add unique Custom CSS files to the child theme so I can have different looks for the same theme on the one domain?

Hope that Makes sense.

Ian