Hello ChicagoMusicGuide,
Thank you so much for using our theme SuperMagPro. If you want to customize parent theme, you have to do it on child theme. Please do all the changes on child theme. But if you only want to add the CSS, you can add the css from customizer. From Customize, go to Layout/Design Option then Custom CSS.
https://doc.acmethemes.com/supermagpro/#WheretowritetheCustomCSS?
Add the custom css here. Please try once and if not worked, let us know.
Best Regards,
Acme Supports