This way we simplify the code a bit.
Note we're only using this function when variables for background colors
are already defined, since that means customizing the variable using the
background color will automatically change the color of the text.
Customization isn't easier when using raw colors.
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.
Note we require extra <span> tags because the <progress> tag is an empty
tag (like <img>), and so it can't have ::before or ::after
pseudo-elements. There's a workaround for that, but currently it only
works on Chrome.
For some reason, the text seems to be slightly misaligned vertically in
all implementations I've tried. So the `top: -0.1rem` rule is a hack to
align it properly.
The selector `nth-child(even)` didn't need specific rules, and it's
easier to understand the code for the selector `nth-child(odd)` if all
breakpoints are grouped together.
