I have inherited an 8.x site using ZURB Foundation as a base theme. However: Foundation somehow got installed twice, once in
docroot/themes/zurb_foundation (8.x-6.0-alpha4) and once in
docroot/themes/contrib/zurb_foundation (8.x-6.0-alpha5). According to the Appearance admin page, the one not in the
contrib folder, 8.x.6.0-alpha4, is the active theme. I suspect that got installed manually, used as the base theme, and then at a later date, someone added the other one via composer.
How do I tell the subtheme to use the zurb_foundation base theme in the contrib folder?