Add 950 Tailwind color shades - #520

Closes #519

Tailwind has 950 shades for each color, i.e. a new darkest shade. See https://tailwindcss.com/docs/customizing-colors Those are very useful for building dark themes. They were missing from twind though.

I just created the PR in my browser, I don't feel like checking out the repo for this simple change, hopefully that's not a deal-breaker here. My branch has "Allow edits by maintainers" so feel free to do whatever needs to be done to merge it 😄

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