Change font size the rems are calculated from?

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So now that I’m finally starting to grok block themes and full site editing, I’m wondering if there’s a way to change the “base” font size that a theme uses to calculate its various font sizes that you can choose from when editing. I vaguely [understand font stuff]) in `themes.json`, but those sizes are in rems and I don’t see a way to set what they are relative to. Is there a way to do this? I’m sort of assuming the base font size comes from core WordPress somewhere.

ETA: To clarify a bit. I’d like to find where WordPress sets this value, and then find a way directly to override or change it.

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