Yep. Timothy Ricks is the on this sort of thing for sure!
REM means the root though, so setting it on the body’s global tag won’t do it, as you’ve already found out. The size needs to be defined on the html element. You can do this with custom code and inject it to the Site settings. Once you’ve added it, you should be able to use 1 REM to get the desired effect.
You might already know this, but I think the users browser/OS settings can override this, so if they have their default size set to 20px, then your REM will be sized accordingly, which is a good and bad part of REM I guess
Some useful links:
Reach out if you get stuck, and welcome to WF forums!
on this sort of thing for sure!