fix: Tuck in H1-H3 letter spacing as discussed in prior design jam

The original conversation which led to #33 (closed) involved some discussion around Space Grotesk (the font used in H1, H2 and H3) and its letter spacing. At the time, it was determined that a -2% letter spacing would be ideal, but that we would table the proper implementation of a percentage-based letter spacing until after the conference call.

That conference call is well over and the ratio for percentage-based letter spacing - "1% = 0.01em" - has been confirmed.

This MR belatedly implements that title letter spacing as requested

Closes #33 (closed).