do you mean copy and paste the css there and from then on make my modifications there?
Yes. For example this could be the original style:
And for example you only changed the '
font-size' style, you can still just copy the whole style into your Custom CSS Manager. By "whole" I mean all of this:
In which case, should I go back to the original and undo my modifications?
You don't have to do this, when the theme updates your modifications to those files will be erased automatically.
Has that plugin now created a Child Theme?
No, the plugin has created a CSS stylesheet that is called into the top of every page on your site.