Hello fd-ambient,
The process of changing fonts of site title is fine on your screenshot. While doing that, the site title fonts is changed on demo site. Have you edited any line of code on theme?
While preview your site, site title and tagline is disabled there. Please enable site title and tagline and view that font family is changed or not.https://doc.acmethemes.com/corporate-plus/#ManageSiteTitle,TaglineandSiteIcon
Please check it, if not work, let us know.
Regards,
Acme Supports