Remove hover function

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:

nav {
 background: pink;
 border: blue;
 font-size: 20px;
}

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:

nav {
 background: pink;
 border: blue;
 font-size: 20px;
}

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.

It’s not entirely clear: are you saying past ONLY the parts that I changed?

Yes, you really don’t want to be wading through a thousand lines of code in your Custom CSS Manager. Plus that’s duplicating a lot of stuff unnecessarily.

In which case, how will the plugin read them and source them properly?

The plugin’s stylesheet will take precedence over Spun’s style.css file. The plugin doesn’t remember where the CSS came from or its position in its original file, as that information isn’t important anymore.

This should help with CSS specificity, but if you’ve edited things like media queries that may mean you’ll have to redeclare those media queries on your styles in the same code section that was in the original CSS.