Matrix field colours are low contrast in high contrast mode

[BeksOmega]

Originally noticed here: #49 (comment)

[cwillisf]

Oof! I see that this isn't new, but I'm not familiar enough with the contrast work to know why it's like this. Specifically, I'm not sure if this is just something we missed in the high-contrast implementation, or if there's some problem with getting all the color options piped through to this code.

I think the important part is that it needs to be possible for us to fix it. Scratch Foundation folks can argue about how to fix it (what colors to choose) in our own time: that shouldn't block Spork efforts.