I didn't include primitive values in the demo, but it doesn't mean you can't use them (even between modes like desktop/mobile).

Font size and line-height could come from the same primitive values. You could t-shirt size from smallest (XXS = 12px) to largest (XXL = 60px). Then, match the primitive variables to their semantic counterparts.

I would recommend setting up an Excel spreadsheet while doing this to make sure you don't miss any values between modes.