We were defining (for instance) white text against the `$brand`
background. That meant that, if somebody customized the `$brand` color
so it used a light color, they had to customize the text color as well
in order to guarantee proper contrast between text and background
colors.
So we're using `color-pick-contrast` instead, which means we don't have
to manually calculate whether white or black will be the color which
makes the text more readable.
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