What parameter should I pass to wp_enqueue_style to depend on the themes stylesheet?

First issue – you have dependency backwards. Depending on something means loading after dependency, while you want earlier.

Second issue – theme’s stylesheet doesn’t actually use enqueue, it is usually codded directly in header.php file of theme. And since it seems to come before wp_head() call – you have no hook to ensure your stylesheet is above it.

So this is commonly handled like your workaround – queued stylesheet with option to disable it by code or option in admin.