Change H1 size only?

Hi,

I have all my text set to the same size and lowercase across my site because I want it that way. Except I now want to make just one heading (h1) all caps. How do I code this so just this heading changes across my site whenever I select text as H1?

Also attached is the current code to make all text the same size.

https://smilodon-ocelot-my5f.squarespace.com/home-1

Thank you!