I modified the CSS for the Default theme. I kept copies of the original files but eventually want to do a lot more.

In order to keep a clean copy, I cloned Default and named DefaultRBC.

When I do updates, I'm assuming that unless reference is made to fixes to Default theme, my custom theme doesn't need touching?

Correct, you're fine as long as you have a separate directory for your custom theme, it won't be overwritten.