R1:84aff44bcd64

Add a "Red/Green Colorblind" accessibility mode, make all web UIs and email respect it

Summary:
Fixes T12172. Fixes T12060. This allows runtime code building CSS for mail to read CSS variables, then makes all the code do that.

It reverts the non-colorblind red/green to the colors in use before T12060, which seem better for non-colorblind users since no one really complained?

Test Plan:
- Viewed code diffs in Web UI.
- Viewed prose diffs in Web UI.
- Viewed code diffs in email.
- Viewed prose…