Subtheming -  what happens with parent theme's style.css
You can choose which files you want to override in the parent theme... its as simple as including a copy of the files using the same name ex style.css just drop that anywhere inside your sub theme and tweak away.  

Any file with the same exact names as files in the parent-theme will now default to the file that is in your sub_theme. So... if you still want the original style.css to load, do not add it to your sub-theme at all &amp; instead create a custom.css and any similarly named style codes will default to the copy in your sub-themes css files first then if needed the original.

To add from your comment below, do not remove the style.css in the main theme unless you are going to bring in all of its code into your custom file.  What would be easiest for light edits would be to create a custom.css file in your sub-theme and only copy over the codes you need to tweak, if your going hog-wild just copy over the whole contents style.css

Hope that helps :)