Focus styling

Some very minor open questions about Input focus underline styling:

• What color should the focus indicator be while animating? (pressed or non-pressed color)
• For borderBottom pressed:
  • The designs for outline/underline show a thick bottom border in the pressed state as the focus indicator animates in, but this isn't included in the animation demoing the focus border (and I can't tell if Windows 11 uses it).
  • This wide border could be added on the root (while maintaining the blue focus border) using :focus-within:before + setting the bottom border color on :focus-within.

Other

The design spec shows a custom text highlight color, but this isn't currently implemented: users can customize system highlight color, which can be an accessibility affordance, so we shouldn't disrupt that (MAS 4.3.1). Also, it's not implemented in either Fluent UI React v8 or Northstar.