Hi Liam,
It's not wise to use the Atlas_Core module for this. Customizing your styling is usually done in a custom styling module or your own theme folder. This folder structure can be found under your App settings:
or the ‘theme’ folder in the File Explorer
It’s best to place your fonts in this folder. This way it won't be deleted when the Atlas_Core module is updated.
Putting your fonts in this folder allows custom-variables.scss to still access them.
Hope this helps!