Hi There @Symbiosis
Thanks for writing in! If you're using V2 button elements in Cornerstone, you can set font family under Primary Text Format section.
Basically, you need to add your preferred fonts into the Font Manager first and then you can select those fonts inside your button's font family option. You can refer to our detailed guide on Font Manager here (https://theme.co/apex/forum/t/font-manager/101).
If you're referring to a classic button,
You can see those inline CSS options when you scroll down to the bottom of the button options.
Hope that helps.