need full url to child theme's custom.css and style.css

Hello — we’re using Enfold with a child theme, and we’re in the process of adding a Wufoo form to our site. Most of the Wufoo form customization is done on Wufoo, but they require a link (the full url) to a stylesheet hosted on our server for more complex styling. We thought we’d use the “Quick Css” field for that. Couple of questions:

1) What is the filename and full url to that file.
2) Is that file part of the child theme or is it overwritten when Enfold is updated?
3) If that doesn’t work, where would you recommend storing a wufoo-specific css file?

Thanks!