After playing around with a clean install of Concrete, I figured out that I can change font settings in default CSS and it actually works (likely also bad practice). I also figured out that trying to mirror those changes to an instance I’ve already been working with doesn’t do anything.
I think that, for this project, I’m just going to start over again from scratch and copy over the custom block templates from my client’s current staging environment after I’ve gotten the base CSS the way I want it. Regardless, many thanks for your input on this!