fix: Modify subnav copy design to match spec
As per the conversation referred to in #23 (closed), and as partially-implemented in !72 (merged). this MR brings the subnav in line with the Figma design:
- The color of "unselected" subnav copy is #556272 ("accent de-emphasized").
- The height of the subnav is 72px.
- The underline matches the width of the text label, rather than the width of the text plus its padding.
- The spacing between each nav item is 2.5rem, which is equivalent to 40px.
Closes #23 (closed)