Please don't take this the wrong way, but you should not be using classic or template default. Any upgrades will wipe out all your hard work and make your life miserable.

There might be one small argument for not using a custom directory, but the only one I can think of is that you will totally abandon your current cart on upgrade.

Anyway, if you do a View Page Source in your browser by right-clicking on your site, you should see what folder the system looks in for the stylesheet.css. There will be a line near the top of the view source that shows it loads the stylesheet.css. That folder should be your custom, but, if you see classic or template_default, that is what you would use in your case.

I'd still investigate the override system in the Tutorials?FAQs section. As Monk says, "You'll thank me later."