This feature could be implemented with a button or checkbox in page settings, where when selecting the page to have its own CSS file, style could be double checked against primary CSS file and copied over to new CSS file. Subsequently, a cleanup of the primary CSS classes could be performed, removing any non-used classes or ids.
Yes, we use webflow to make promo LPS, each lp has its own design. We have at leas 100 lps as of now on the project. The site css is giant.