@creedon It sort of worked, but not entirely. I implemented the v.8 code. While the second button does now take on the tertiary styling, it seems the code has broken the font that's used in the header. It now presents as a fall-back font, and not the site font (Poppins, in this case). Any ideas why that might be?
EDIT: Having a look at Twitter, it looks like the custom font issue is being experienced by other users. Perhaps this is a Squarespace issue.